Due to the refactoring in !16082, `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