Mono: Some StackTrace to StackInfo[] fixes

- Sometimes `StackFrame.GetMethod()` returns null (e.g.: latest frame of a `MissingMethodException`). Still not sure what to do with that frame (maybe skip it), but at least it no longer fails.
- Skip `CSharpLanguage::debug_get_current_stack_info()` if an error is printed from `GDMonoUtils::update_corlib_cache()`.
- Fix crash when calling `GDMonoUtils::print_unhandled_exception(exc)` if there is no ScriptDebugger attached.

modules/mono/glue/cs_files/DebuggingUtils.cs

@@ -26,6 +26,12 @@ namespace Godot
 
             MethodBase methodBase = frame.GetMethod();
 
+            if (methodBase == null)
+            {
+                methodDecl = string.Empty;
+                return;
+            }
+
             StringBuilder sb = new StringBuilder();
 
             if (methodBase is MethodInfo methodInfo)