Notification: ensure arg is StructureType when decoding ImageHint

If `arg` is any other type than `StructureType`, broken imagehint would crash plasmashell.
This change checks for the imagehint that it is correct type.

It is better to return no image at all than crash whole shell.

Tested with the ruby code attachment in the bug report.

BUG:481033


(cherry picked from commit 390d4fa6)