lib/model: Fix race in GetIgnores (fixes #4300)

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In addition this function returned an error when .stignore file was not
present, which is perfectly valid. Also removed inconsistent nil check in
ignores.go (only relevant for tests) and adjusted walk.go to do what it says
in comments (check if Matcher is nil) to prevent nil deref in tests.

Originally reported in:
https://forum.syncthing.net/t/reason-for-panic-maybe-too-little-ram/10346

Regression from #3996

GitHub-Pull-Request: https://github.com/syncthing/syncthing/pull/4301

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