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

dvc/commands/experiments/diff.py

@@ -27,7 +27,7 @@ class CmdExperimentsDiff(CmdBase):
 
         if self.args.json:
             ui.write_json(diff)
-        else:
+        elif diff:
             from dvc.compare import show_diff
 
             precision = self.args.precision or DEFAULT_PRECISION

dvc/repo/experiments/show.py

@@ -142,6 +142,9 @@ def show(
     fetch_running: bool = True,
 ):
 
+    if repo.scm.no_commits:
+        return {}
+
     if onerror is None:
         onerror = onerror_collect
 

dvc/repo/plots/diff.py

@@ -12,6 +12,8 @@ def _revisions(repo, revs, experiment):
 
 
 def diff(repo, *args, revs=None, experiment=False, **kwargs):
+    if repo.scm.no_commits:
+        return {}
     return repo.plots.show(
         *args, revs=_revisions(repo, revs, experiment), **kwargs
     )

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": {}}
 

tests/func/experiments/test_show.py

@@ -35,6 +35,18 @@ def make_executor_info(**kwargs):
     return ExecutorInfo(**kwargs)
 
 
+@pytest.mark.vscode
+def test_show_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.show() == {}
+
+
 @pytest.mark.vscode
 def test_show_simple(tmp_dir, scm, dvc, exp_stage):
     assert dvc.experiments.show()["workspace"] == {

tests/func/plots/test_diff.py

@@ -1,3 +1,5 @@
+import pytest
+
 from tests.utils.plots import get_plot
 
 
@@ -46,3 +48,15 @@ def test_diff_dirty(tmp_dir, scm, dvc, run_copy_metrics):
     assert get_plot(
         diff_result, "workspace", "definitions", file="", endkey="data"
     ) == {"metric.json": props}
+
+
+@pytest.mark.vscode
+def test_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().plots.diff() == {}