bls: fix is_infinity when aggregating onto empty AggregateSignature

[Galoretka]

• Ensure is_infinity reflects the actual aggregate when aggregating onto an empty AggregateSignature.
• Previously, starting from empty() and aggregating an infinity signature produced an aggregate whose underlying point was infinity but is_infinity stayed false. This broke eth_fast_aggregate_verify for empty pubkeys and desynced the flag from the serialized value.
• Update add_assign and add_assign_aggregate: when self was empty, set is_infinity = other.is_infinity; otherwise keep the existing conjunction behavior. This aligns with the doc that empty acts as “set to infinity before aggregation” and keeps is_infinity consistent with the bytes.

[cla-assistant]

Thank you for your submission! We really appreciate it. Like many open source projects, we ask that you sign our Contributor License Agreement before we can accept your contribution.
_{You have signed the CLA already but the status is still pending? Let us recheck it.}

[cla-assistant]

Thank you for your submission! We really appreciate it. Like many open source projects, we ask that you sign our Contributor License Agreement before we can accept your contribution.
_{You have signed the CLA already but the status is still pending? Let us recheck it.}

[mergify]

Some required checks have failed. Could you please take a look @Galoretka? 🙏