Fix release stage filtering on cocoa platforms
Fix release stage filtering on cocoa platforms

During delivery, if notify release stages is non-null, then reports are
only sent if release stage is in notify release stages. On cocoa, the
value defaults to nil, but was parsed into an empty array, so no reports
could ever be sent.

bugsnag/bugsnag-unity,bugsnag/bugsnag-unity,bugsnag/bugsnag-unity,bugsnag/bugsnag-unity,bugsnag/bugsnag-unity,bugsnag/bugsnag-unity