Draft: Update LabelFinder to use index more efficiently

What does this MR do?

In !61648 (comment 594987131), @dskim_gitlab made the observation that we should be able to use the index more efficiently when querying labels:

What do you think about specifying project_id IS NULL and group_id IS NULL when querying group_id or project_id to utilize existing unique index? It seems that group_id and project_id are mutually exclusive as far as I can tell so it shouldn't change any underlying behavior, but I'm not sure if I missed anything.

This MR makes that optimization.

See also !61648 (comment 596152850) and !61648 (comment 596162004) for additional observations.

Related to #339349

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