Fix error 500 when viewing commit and merge request diffs

Due to the refactoring in !16082 (merged), Blob#batch no longer falls back to a default blob_size_limit. Since Repository#batch_blobs was using a default nil value, this would cause issues in the Blob#find_by_rugged method.

This fix here is to be consistent and use a non-nil default value in Repository#batch_blobs.

The problem was masked in development and tests because Gitaly is always enabled by default for all features.

Closes #41735 (closed)