Fix EventIndexProducer when AMITAG has a numeric value

An AMITAG processing at tier0 was incorrectly assigned a value "1450" instead of "f1450".
EventIndex producer code treats the incorrect value as a int instead of str, so the transformation fails due to a type error:
TypeError: Cannot set EventIndex.BeginGUID_p1.AMITag to 1450: 1450 has type <class 'int'>, but expected one of: (<class 'bytes'>, <class 'str'>)
This patch protects the failed assignment with a string cast to avoid the error. Just one EventIndexProducer package file should be affected by this patch.