Fix panic in recursive cache (#16298)

Fix panic in recursive cache (#16298)

There is a bug with last commit cache recursive cache where the last
commit information that refers to the current tree itself will cause a
panic due to its path ("") not being included in the expected tree entry
paths.

This PR fixes this by skipping the missing entry.

Fix #16290

Signed-off-by: Andrew Thornton <art27@cantab.net>
Co-authored-by: 6543 <6543@obermui.de>
Co-authored-by: techknowlogick <techknowlogick@gitea.io>