C#: Various fixes to generic scripts

- Report a diagnostic when there are multiple classes that match the script file name in the same script since that will result in a duplicate path key in the bimap and it's not allowed.
- Fix InspectorPlugin to handle empty paths in case the project was built with a previous version of Godot that used empty paths for generic scripts.
- Add tests for the new diagnostic GD0003.

modules/mono/editor/Godot.NET.Sdk/Godot.SourceGenerators/Common.cs

@@ -65,6 +65,16 @@ namespace Godot.SourceGenerators
                 outerTypeDeclSyntax.SyntaxTree.FilePath));
         }
 
+        public static readonly DiagnosticDescriptor MultipleClassesInGodotScriptRule =
+            new DiagnosticDescriptor(id: "GD0003",
+                title: "Found multiple classes with the same name in the same script file",
+                messageFormat: "Found multiple classes with the name '{0}' in the same script file",
+                category: "Usage",
+                DiagnosticSeverity.Error,
+                isEnabledByDefault: true,
+                "Found multiple classes with the same name in the same script file. A script file must only contain one class with a name that matches the file name.",
+                helpLinkUri: string.Format(_helpLinkFormat, "GD0003"));
+
         public static readonly DiagnosticDescriptor ExportedMemberIsStaticRule =
             new DiagnosticDescriptor(id: "GD0101",
                 title: "The exported member is static",