Allow an empty metadata cache when adding lower bounds (#21225)
Follow-up to #21163.
Reusing a metadata-free lock can leave the in-memory distribution index
empty. When `uv add` inserts a lower bound and locks again, invalidating
the project's metadata must tolerate an entry that was never cached.
Remove the cache-presence assertion and extend the existing lower-bound
regression test to cover adding a redundant marker after reusing a
metadata-free lock.
