Fix handling of empty parameters in parameter_overview.

The test should not be on whether it is an empty dict, but whether after flattening it is empty.
Without this fix the added test fails with:

```
  File "../py/clu/parameter_overview.py", line 156, in _get_parameter_rows
    names, values = map(list, tuple(zip(*sorted(params.items()))))
    ^^^^^^^^^^^^^
ValueError: not enough values to unpack (expected 2, got 0)
```
PiperOrigin-RevId: 715237667

clu/parameter_overview.py

@@ -165,8 +165,8 @@ def _get_parameter_rows(
         f"Expected `params` to be a dictionary but got {type(params)}"
     )
 
+  params = flatten_dict(params)
   if params:
-    params = flatten_dict(params)
     names, values = map(list, tuple(zip(*sorted(params.items()))))
   else:
     names, values = [], []

clu/parameter_overview_test.py

@@ -92,6 +92,8 @@ def test_count_parameters(self):
   def test_get_parameter_overview_empty(self):
     self.assertEqual(EMPTY_PARAMETER_OVERVIEW,
                      parameter_overview.get_parameter_overview({}))
+    self.assertEqual(EMPTY_PARAMETER_OVERVIEW,
+                     parameter_overview.get_parameter_overview({"a": {}}))
 
   def test_get_parameter_overview(self):
     rng = jax.random.PRNGKey(42)