Core: `ignore_unavailable` shouldn't ignore closed indices

The `ignore_unavailable` request setting shouldn't ignore closed indices if a single index is specified in a search or broadcast request.

PR for #7153
Actually, I did some digging, this if was introduced with #6475, and I think its purpose was to check that state of indices  after aliases resolution. This is a bug that we should address on a separate issue, that should be about index state checks when resolving aliases, since it seems we don't check that at all at this time.