repo: Handle no commits for exp show and plots diff.

Make it consistent with all the other commands that use `brancher` (i.e. `dvc diff`, `metrics diff`, `params diff`)

Closes #8163

tests/func/experiments/test_diff.py

@@ -1,6 +1,17 @@
 from funcy import first
 
 
+def test_diff_no_commits(tmp_dir):
+    from scmrepo.git import Git
+
+    from dvc.repo import Repo
+
+    git = Git.init(tmp_dir.fs_path)
+    assert git.no_commits
+
+    assert Repo.init().experiments.diff() == {}
+
+
 def test_diff_empty(tmp_dir, scm, dvc, exp_stage):
     assert dvc.experiments.diff() == {"params": {}, "metrics": {}}