lakefs: during file operations, skip merge when 0 changes

# Description

**Use Case**
If the branch has no commited files/zero changes, don't merge branch, so that we don't get an empty commit.

Should happen before: https://github.com/delta-io/delta-rs/blob/9dc8f328859b4fe66fa82a636aae662cee22db30/crates/lakefs/src/logstore.rs#L161-L163

**Related Issue(s)**
