fix phase_by protocol for tagged operations (#6792)

`cirq.phase_by` can return a default value for operation that cannot be phased,
but that was broken for tagged operations.  Here is a fix for that.

Fixes #6791

cirq-core/cirq/ops/raw_types.py

@@ -939,7 +939,9 @@ def _trace_distance_bound_(self) -> float:
         return protocols.trace_distance_bound(self.sub_operation)
 
     def _phase_by_(self, phase_turns: float, qubit_index: int) -> 'cirq.Operation':
-        return protocols.phase_by(self.sub_operation, phase_turns, qubit_index)
+        return protocols.phase_by(
+            self.sub_operation, phase_turns, qubit_index, default=NotImplemented
+        )
 
     def __pow__(self, exponent: Any) -> 'cirq.Operation':
         return self.sub_operation**exponent

cirq-core/cirq/ops/raw_types_test.py

@@ -662,6 +662,8 @@ def test_tagged_operation_forwards_protocols():
     assert cirq.commutes(tagged_x, clifford_x)
     assert cirq.commutes(clifford_x, tagged_x)
     assert cirq.commutes(tagged_x, tagged_x)
+    assert cirq.phase_by(clifford_x, 0.125, 0, default=None) is None
+    assert cirq.phase_by(tagged_x, 0.125, 0, default=None) is None
 
     assert cirq.trace_distance_bound(y**0.001) == cirq.trace_distance_bound(
         (y**0.001).with_tags(tag)