fix: prevent unnecessary error throwing in stringifyRule

[megboehlert]

When a css import rule was being stringified (in stringifyRule) and the rule didn't have a styleSheet property at time of processing, an error would be thrown and passed up to stringifyStyleSheet where it was caught. Once caught, it would set null for the attributes._cssText property of the serializedNode. When watching the replay, these style elements would be empty instead of populated with the correct cssText.