Add warning / check for weighted vertex groups in Merge Bone to Active

[Vianvolaeus]

Not sure if there's any niche functionality to merge empty bones / non-weighted bones to active in this context, but currently bones do not merge if they're not weighted.

We can either:

a) check for weights and refuse to run, etc / add warning

b) not implement this functionality. Simply state weighted bones only for operator

Leaning towards b) currently unless I can think of a good reason to check the alternative. I don't see why you'd need to merge non weighted bones to another non-weighted bone. The correct course of action in that context is probably just to delete - there's no data to transfer down the chain. Maybe there's some animation rigging uses I'm not aware of, but I don't think so..

[Vianvolaeus]

Need to double check this behaviour. I think it was fixed or accounted for