No real requirement to ground the mast of an indoor antenna in the NEC (electrical code), other than the generic requirement to ground "electrically conductive material that are likely to become energized". But I think it's pretty unlikely for a mast in most attics to become energized.
Note the code grounding requirement is mainly in place to protect people, not equipment.
One reason for grounding outside antennas is to bleed off static electricity buildup caused by wind. This should not be an issue with an attic antenna, so I wouldn't bother grounding an attic antenna.
I would NOT leave an outdoor antenna ungrounded or improperly grounded (not bonded to power).
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