Django 5.1 removes support for `Meta.index_together` in favor of just including these in `Meta.indexes`. A previous change updated signature calculation and existing unit tests for this, but one thing I wanted to make sure continued to work was an evolution which removes `index_together` and adds `indexes`, even when running on Django>=5.1. This change adds a test which will manually modify the signature (on affected versions) to simulate this case, and verify that we end up correctly modifying the signature while generating empty SQL (since there's no change in the actual indexes). Testing Done: Ran new test on all versions and database types.