Fix dep.get_variable() with empty string default_value

mesonbuild/dependencies/pkgconfig.py

@@ -402,7 +402,7 @@ class PkgConfigDependency(ExternalDependency):
             if not variable:
                 ret, out, _ = self._call_pkgbin(['--print-variables', self.name])
                 if not re.search(r'^' + variable_name + r'$', out, re.MULTILINE):
-                    if default:
+                    if default is not None:
                         variable = default
                     else:
                         mlog.warning(f"pkgconfig variable '{variable_name}' not defined for dependency {self.name}.")

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
+      }
+    ]
+  }