Fix three nil dereference issues in the policy resolution code:
- newResolvedAddresses: preserve partial IP results when errors occur
instead of discarding valid IPSets. Callers already handle errors
and nil results independently, so returning both allows partial
resolution (e.g. groups with phantom users) to work correctly.
- resolveTagOwners: guard against nil ResolvedAddresses before calling
Prefixes(), since Resolve may return nil when resolution fails.
- Asterix.resolve: guard against nil *Policy pointer, which occurs
when resolving wildcards without a policy context (e.g. in tests).
Updates #2180
5f3bddc663