add test for metrics without valid input

tests/taskmodules/test_re_text_classification_with_indices.py

@@ -1813,5 +1813,20 @@ def test_configure_model_metric(documents, taskmodule):
         },
     )
 
+    # no targets and no predictions
+    metric.reset()
+    no_targets = {"labels": torch.tensor([0, 0, 0])}
+    no_predictions = {"labels": torch.tensor([0, 0, 0])}
+    metric.update(no_targets, no_predictions)
+    state = get_metric_state(metric)
+
+    # TODO: assert state
+    assert state
+    # TODO: assert metric.compute()
+    torch.testing.assert_close(
+        metric.compute(),
+        {},
+    )
+
     # ensure that the metric can be pickled
     pickle.dumps(metric)