Clarify PipelineStepTester output for empty suffix.

The original output format assumed a non-empty suffix, but this leads
to a confusing "()" if the full pipeline is tested at once.

python/lsst/pipe/base/tests/pipelineStepTester.py

@@ -157,7 +157,10 @@ def run(self, butler: Butler, test_case: unittest.TestCase) -> None:
                 butler.registry.registerDatasetType(node.dataset_type)
 
         if not pure_inputs.keys() <= self.expected_inputs:
-            missing = [f"{k} ({pure_inputs[k]})" for k in pure_inputs.keys() - self.expected_inputs]
+            missing = []
+            for type_name in pure_inputs.keys() - self.expected_inputs:
+                suffix = pure_inputs[type_name]
+                missing.append(type_name + (f" ({suffix})" if suffix else ""))
             raise AssertionError(f"Got unexpected pure_inputs: {missing}")
 
         if not all_outputs.keys() >= self.expected_outputs: