Unparsing: fix handling of missing fields for recurse_field

When instantiating a "node" configuration that contains recurse_field
templates, Langkit should keep placeholder templates when processing
recurse_field templates for "missing fields" (null field, or an empty
list that unparsing must treat as a missing field).

Before this fix, the engine did not have such placeholder, and so did
not have enough templates to build the vector of template arguments for
all fields, leading to Constraint_Error exceptions.

Author: pmderodat

langkit/support/langkit_support-generic_api-unparsing.adb

@@ -1681,6 +1681,9 @@ package body Langkit_Support.Generic_API.Unparsing is
 
                            Arguments.Field_Docs.Append
                              (Pool.Create_List (Items));
+                        else
+                           Arguments.Field_Docs.Append
+                             (Pool.Create_Empty_List);
                         end if;
                      end;
                   end loop;

testsuite/tests/ada_api/unparsing/cmd_recurse_field.json

@@ -1,5 +1,19 @@
 {
   "node_configs": {
+    "ParamSpec": {
+      "node": [
+        {"kind": "recurse_field", "field": "f_name"},
+        "softline",
+        {"kind": "text", "text": ":"},
+        "softline",
+        {"kind": "recurse_field", "field": "f_type_expr"},
+        "line",
+        {"kind": "recurse_field", "field": "f_default_expr"}
+      ],
+      "fields": {
+        "f_default_expr": ["line", "recurse"]
+      }
+    },
     "VarDecl": {
       "node": [
         {"kind": "recurse_field", "field": "f_is_null"},

testsuite/tests/ada_api/unparsing/commands.adb

@@ -107,7 +107,10 @@ begin
    Check ("cmd_literalline.json");
    Check ("cmd_markasroot.json");
    Check ("cmd_recurse.json");
-   Check ("cmd_recurse_field.json");
+   Check
+     ("cmd_recurse_field.json",
+      "var i: Int = 0;" & ASCII.LF
+      & "def f(i: Int): Int {i;}");
    Check
      ("cmd_recurse_flatten.json",
       "var i: Int = AAAAAAAAAAAAAAAAAA"

testsuite/tests/ada_api/unparsing/test.out

@@ -636,7 +636,46 @@ Done.
     },
     {
       "kind": "text",
-      "text": ";"
+      "text": ";def f(i"
+    },
+    {
+      "kind": "command",
+      "command": {
+        "command": "line",
+        "literal": false,
+        "soft": true,
+        "hard": false
+      }
+    },
+    {
+      "kind": "text",
+      "text": ":"
+    },
+    {
+      "kind": "command",
+      "command": {
+        "command": "line",
+        "literal": false,
+        "soft": true,
+        "hard": false
+      }
+    },
+    {
+      "kind": "text",
+      "text": "Int"
+    },
+    {
+      "kind": "command",
+      "command": {
+        "command": "line",
+        "literal": false,
+        "soft": false,
+        "hard": false
+      }
+    },
+    {
+      "kind": "text",
+      "text": "):Int{i;}"
     }
   ]
 }