[Attributor][FIX] Deal with implicit `undef` in AAPotentialConstantValues.

In contrast to AAPotentialValues, the constant values version can
contain implicit `undef` in the set. We had an assertion that could
misfire before. Handle it properly now.

GitOrigin-RevId: f81a209337bbcf527e0b4adfcfedb44d798ac91d
1 file changed