Avoid PRECONDITION_FAILURE on editing an empty file

Fix #1435

Author: reznikmm

source/ada/lsp-ada_completions-keywords.adb

@@ -46,6 +46,7 @@ package body LSP.Ada_Completions.Keywords is
    begin
       if Filter.Is_End_Label or else
         Filter.Is_Numeric_Literal or else
+        Node.Is_Null or else
         not Libadalang.Common.Is_Trivia (Prev)
       then
          --  Propose keyword completion if we are not within an end label

testsuite/ada_lsp/completion.null_node/default.gpr

@@ -0,0 +1,2 @@
+project Default is
+end Default;

testsuite/ada_lsp/completion.null_node/main.adb

@@ -0,0 +1,2 @@
+--
+--

testsuite/ada_lsp/completion.null_node/test.json

@@ -0,0 +1,188 @@
+[
+   {
+      "comment": ["test PRECONDITION_FAILURE on an empty file editing"]
+   },
+   {
+      "start": {
+         "cmd": ["${ALS}"]
+      }
+   },
+   {
+      "send": {
+         "request": {
+            "jsonrpc": "2.0",
+            "id": 1,
+            "method": "initialize",
+            "params": {
+               "processId": 199714,
+               "rootUri": "$URI{.}",
+               "capabilities": {
+                  "workspace": {
+                     "applyEdit": true
+                  },
+                  "textDocument": {
+                     "completion": {
+                        "completionItem": {
+                           "snippetSupport": true,
+                           "documentationFormat": ["markdown", "plaintext"]
+                        }
+                     }
+                  },
+                  "window": {
+                     "workDoneProgress": true
+                  }
+               }
+            }
+         },
+         "wait": [
+            {
+               "jsonrpc": "2.0",
+               "id": 1,
+               "result": {
+                  "capabilities": {
+                     "textDocumentSync": 2,
+                     "completionProvider": {
+                        "triggerCharacters": [".", ",", "'", "("],
+                        "resolveProvider": true
+                     }
+                  }
+               }
+            }
+         ]
+      }
+   },
+   {
+      "send": {
+         "request": {
+            "jsonrpc": "2.0",
+            "method": "initialized",
+            "params": {}
+         },
+         "wait": []
+      }
+   },
+   {
+      "send": {
+         "request": {
+            "jsonrpc": "2.0",
+            "method": "textDocument/didOpen",
+            "params": {
+               "textDocument": {
+                  "uri": "$URI{main.adb}",
+                  "languageId": "ada",
+                  "version": 1,
+                  "text": "--\n--\n"
+               }
+            }
+         },
+         "wait": []
+      }
+   },
+   {
+      "send": {
+         "request": {
+            "jsonrpc": "2.0",
+            "method": "workspace/didChangeConfiguration",
+            "params": {
+               "settings": {
+                  "ada": {
+                     "projectFile": "default.gpr"
+                  }
+               }
+            }
+         },
+         "wait": [
+            {
+               "method": "$/progress",
+               "params": {
+                  "token": "<ANY>",
+                  "value": {
+                     "kind": "end"
+                  }
+               }
+            }
+         ]
+      }
+   },
+   {
+      "send": {
+         "request": {
+            "jsonrpc": "2.0",
+            "method": "textDocument/didChange",
+            "params": {
+               "textDocument": {
+                  "uri": "$URI{main.adb}",
+                  "version": 2
+               },
+               "contentChanges": [
+                  {
+                     "range": {
+                        "start": {
+                           "line": 2,
+                           "character": 0
+                        },
+                        "end": {
+                           "line": 2,
+                           "character": 0
+                        }
+                     },
+                     "rangeLength": 0,
+                     "text": "p"
+                  }
+               ]
+            }
+         },
+         "wait": []
+      }
+   },
+   {
+      "send": {
+         "request": {
+            "jsonrpc": "2.0",
+            "id": 16,
+            "method": "textDocument/completion",
+            "params": {
+               "textDocument": {
+                  "uri": "$URI{main.adb}"
+               },
+               "position": {
+                  "line": 2,
+                  "character": 1
+               },
+               "context": {
+                  "triggerKind": 1
+               }
+            }
+         },
+         "wait": [
+            {
+               "id": 16,
+               "result": {
+                  "isIncomplete": false,
+                  "items": []
+               }
+            }
+         ]
+      }
+   },
+   {
+      "send": {
+         "request": {
+            "jsonrpc": "2.0",
+            "id": 44,
+            "method": "shutdown"
+         },
+         "wait": [
+            {
+               "id": 44,
+               "result": null
+            }
+         ]
+      }
+   },
+   {
+      "stop": {
+         "exit_code": 0
+      }
+   }
+]

testsuite/ada_lsp/completion.null_node/test.yaml

@@ -0,0 +1 @@
+title: 'completion.null_node'