Remove zero object ID workaround in user_commit_files

Problem

target_sha was not provided when creating the branch because Gitaly did not support zero object IDs. See thread (!161981 (comment 2038365067)).

If set, it must either contain a valid, full object ID or the zero object ID in case the branch should be created. source

The Gitaly issue was resolved in gitaly#6290 (closed).

Proposal

Provide blank_ref to target_sha when the branch_name is not created yet.