Parse: Fix `ALTER INDEX my_index_name` to return `tables = []`

[ayufan]

When parsing ALTER INDEX index_name SET (fast_update = true); the pg_query wrongly classifies index_name as a table name.

Expected output for above is tables = [].

This fix is targeted to only ignore OBJECT_INDEX to avoid breaking all other patterns.

References:

• Resolves: https://github.com/pganalyze/pg_query/issues/284
• PostgreSQL documentation: https://www.postgresql.org/docs/current/sql-alterindex.html
• Related to: https://gitlab.com/gitlab-org/gitlab/-/merge_requests/120838#note_1391849262

[ayufan]

@lfittl If possible could we push a new release? Of course: I can help out with that :)

[lfittl]

@ayufan 4.2.1 is now released!