Fix mixed variant downloading

[DN6]

What does this PR do?

Oversight on my part when merging #11587 which fixes the case where only variant safetensors are present in a repo.

I missed that we always need to fallback to checking for non-variants if no variants are present otherwise mixed variant loading will break. Added a test to catch this case.

Fixes # (issue)

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[sayakpaul]

Tests look good too!