test: Add guard when no commit needs testing

Without this patch, github actions fail.

It's a temporary workaround until [1] is done.

[1]: https://github.com/purpleidea/mgmt/issues/643

test/test-commit-message.sh

@@ -134,12 +134,12 @@ then
 		head=""
 	fi
 	commits=$(git log --no-merges --format=%H origin/${ref}..${head})
-	[[ -n "$commits" ]]
-
+	if [[ -n "$commits" ]]; then
 		for commit in $commits
 		do
 			test_commit_message $commit
 			test_commit_message_common_bugs $commit
 		done
+	fi
 fi
 echo 'PASS'