Fix EngineProgram.get_circuit(_async) empty code detection

Author: maffoo

cirq-google/cirq_google/engine/engine_program.py

@@ -361,7 +361,9 @@ async def get_circuit_async(self) -> cirq.Circuit:
         Returns:
             The program's cirq Circuit.
         """
-        if self._program is None or self._program.code is None:
+        # The code field is an any_pb2.Any and is always set. But if the program has not
+        # been fetched this field may be empty, which we can see by checking the type_url.
+        if self._program is None or not self._program.code or not self._program.code.type_url:
             self._program = await self.context.client.get_program_async(
                 self.project_id, self.program_id, True
             )