Check if ii_AmCache is NULL in aminsertcleanup

Check if ii_AmCache is NULL in aminsertcleanup

Fix a bug introduced by c1ec02be1d79. It may happen that the executor
opens indexes on the result relation, but no rows end up being inserted.
Then the index_insert_cleanup still gets executed, but passes down NULL
to the AM callback. The AM callback may not expect this, as is the case
of brininsertcleanup, leading to a crash.

Fixed by only calling the cleanup callback if (ii_AmCache != NULL). This
way the AM can simply assume to only see a valid cache.

Reported-by: Richard Guo
Discussion: https://postgr.es/m/CAMbWs4-w9qC-o9hQox9UHvdVZAYTp8OrPQOKtwbvzWaRejTT=Q@mail.gmail.com