Added attributes existence check in self::DELETE

Using PodioItem::delete with an empty array as the attributes argument
results in a Fatal error: Uncaught PodioBadRequestError: "Invalid value
"1?" (string): must be {'1', 'on', 'true', 'y', 'yes', True, '0',
'false', 'n', 'no', 'off', False}” due to the extra question mark being
inserted. This solves the problem.