Gateset: fix invalid repr when gateset is empty (quantumlib#5322)

Fixes quantumlib#5197

@tanujkhattar

Author: verult

cirq-core/cirq/ops/gateset.py

@@ -525,9 +525,10 @@ def _value_equality_values_(self) -> Any:
 
     def __repr__(self) -> str:
         name_str = f'name = "{self.name}", ' if self.name is not None else ''
+        gates_str = f'{self._gates_repr_str}, ' if len(self._gates_repr_str) > 0 else ''
         return (
             f'cirq.Gateset('
-            f'{self._gates_repr_str}, '
+            f'{gates_str}'
             f'{name_str}'
             f'unroll_circuit_op = {self._unroll_circuit_op})'
         )

cirq-core/cirq/ops/gateset_test.py

@@ -255,11 +255,12 @@ def test_gateset_init():
     )
 
 
-def test_gateset_repr_and_str():
-    cirq.testing.assert_equivalent_repr(gateset)
-    assert gateset.name in str(gateset)
-    for gate_family in gateset.gates:
-        assert str(gate_family) in str(gateset)
+@pytest.mark.parametrize('g', [gateset, cirq.Gateset(name='empty gateset')])
+def test_gateset_repr_and_str(g):
+    cirq.testing.assert_equivalent_repr(g)
+    assert g.name in str(g)
+    for gate_family in g.gates:
+        assert str(gate_family) in str(g)
 
 
 @pytest.mark.parametrize(