dvcfs: handle repo root

Fixes #7407

dvc/fs/dvc.py

@@ -44,7 +44,7 @@ class DvcFileSystem(FileSystem):  # pylint:disable=abstract-method
 
         fs_key = "repo"
         key = self.path.parts(path)
-        if key == (".",):
+        if key == (".",) or key == ("",):
             key = ()
 
         return (fs_key, *key)

tests/unit/fs/test_dvc.py

@@ -10,6 +10,20 @@ from dvc.hash_info import HashInfo
 from dvc.utils.fs import remove
 
 
+@pytest.mark.parametrize(
+    "path, key",
+    [
+        ("", ("repo",)),
+        (".", ("repo",)),
+        ("foo", ("repo", "foo")),
+        (os.path.join("dir", "foo"), ("repo", "dir", "foo")),
+    ],
+)
+def test_get_key(tmp_dir, dvc, path, key):
+    fs = DvcFileSystem(repo=dvc)
+    assert fs._get_key(path) == key
+
+
 def test_exists(tmp_dir, dvc):
     tmp_dir.gen("foo", "foo")
     dvc.add("foo")