Fix getID method to handle null values and always return a string

[Axiome974]

getID() should always return a string, but in some cases it returns an integer.

[derpixler]

@Axiome974 Thanks for the PR! The fix looks good -- the explicit (string) cast and null check solve the type inconsistency from json_decode() where IDs come back as integers.

Two small suggestions:

1. Please update the docblock to @return string|null since the method now explicitly returns null for unsaved objects.
2. Consider adding a simple test for getID() (e.g., returns null for an unsaved object, returns a string after fetch).