Add test for empty string default_value

Backported from 0185f2e, which contains
a fix that doesn't need to be backported, but we can backport the test
to ensure that it's still working in the 0.61 branch.

Author: nirbheek

test cases/common/211 dependency get_variable method/meson.build

@@ -19,6 +19,7 @@ else
         'pkg-config didn\'t get default when we should have.')
   assert(dep.get_variable(pkgconfig : 'prefix', default_value : default) != default,
         'pkg-config got default when we shouldn\'t have.')
+  assert(dep.get_variable(pkgconfig : 'pkgvarnotfound', default_value : '') == '')
 endif
 
 dep_ct = dependency('llvm', method : 'config-tool', required : false)

test cases/common/211 dependency get_variable method/test.json

@@ -0,0 +1,9 @@
+{
+    "stdout": [
+      {
+        "line": ".*pkgvarnotfound.*",
+        "match": "re",
+        "count": 0
+      }
+    ]
+  }