fix(ollama): guard nil filter in galleryop.ListModels (#9817) (#9836)

The Ollama /api/tags handler passes a nil filter to galleryop.ListModels.
When ModelsPath contains any non-skipped loose file the function then
calls filter(name, nil) and panics, which Echo surfaces to clients as
"Server disconnected without sending a response" - the exact failure
Home Assistant's Ollama integration reports against LocalAI.

Mirror the nil guard already present in
ModelConfigLoader.GetModelConfigsByFilter so every caller is safe, and
add a regression test that exercises the loose-file path with a nil
filter.

Assisted-by: claude:claude-opus-4-7 [Claude Code]

Signed-off-by: Ettore Di Giacinto <mudler@localai.io>
Co-authored-by: Ettore Di Giacinto <mudler@localai.io>