Fix String.GetExtension() return value.

It previously returned the string itself when it should return an empty string according to the docs/examples and the C++ counterpart String::get_extension().

modules/mono/glue/GodotSharp/GodotSharp/Core/StringExtensions.cs

@@ -430,8 +430,8 @@ namespace Godot
         {
             int pos = instance.RFind(".");
 
-            if (pos < 0)
+            if (pos < 0 || pos < Math.Max(instance.RFind("/"), instance.RFind("\\")))
-                return instance;
+                return string.Empty;
 
             return instance.Substring(pos + 1);
         }