Fix AttributeError crash in user_defaults artifact parsing

[prnndk]

When processing artifacts, the user_defaults module crashed with an AttributeError: 'str' object has no attribute 'items'.

This occurs because, in some instances, the plist loader returns a string (e.g., when a previous exception handler sets the plist content to "INVALID FILE") or fails to load a dictionary structure, but the code attempts to iterate over it using .items() without validation.

Changes Made

Added Type Validation: Implemented isinstance(plist, dict) checks before attempting to iterate through plist.items(). This ensures that only valid dictionary objects are processed.

Enhanced Error Handling: Wrapped the file processing loop in a try-except block to gracefully handle InvalidFileException or other generic exceptions.

Safety Checks: Added a check for MCMMetadataIdentifier to ensure the plist contains the necessary metadata before processing, preventing potential KeyErrors.

Test Result: