Fix over-optimistic caching in fetch_array_arg_replace_nulls().

Fix over-optimistic caching in fetch_array_arg_replace_nulls().

When I rewrote this in commit 56a79a869bedc4bf6c35853642694cc0b0594dd2,
I forgot that it's possible for the input array type to change from one
call to the next (this can happen when applying the function to
pg_statistic columns, for instance).  Fix that.