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.Sample/Generic1T.cs

@@ -0,0 +1,9 @@
+#pragma warning disable CS0169
+
+namespace Godot.SourceGenerators.Sample
+{
+    partial class Generic1T<T> : GodotObject
+    {
+        private int _field;
+    }
+}