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.Tests/TestData/GeneratedSources/Generic(Of T)_ScriptPath.generated.cs

@@ -1,5 +1,5 @@
 using Godot;
 [ScriptPathAttribute("res://Generic.cs")]
-partial class Generic
+partial class Generic<T>
 {
 }

modules/mono/editor/Godot.NET.Sdk/Godot.SourceGenerators.Tests/TestData/GeneratedSources/NamespaceA.SameName_ScriptPath.generated.cs

@@ -0,0 +1,9 @@
+using Godot;
+namespace NamespaceA {
+
+[ScriptPathAttribute("res://SameName.cs")]
+partial class SameName
+{
+}
+
+}