Do not send run IDs with UpdateWithStart requests

[fixup after rebase]

Author: dandavison

temporalio/client.py

@@ -6018,14 +6018,21 @@ async def _build_update_workflow_execution_request(
         input: Union[StartWorkflowUpdateInput, UpdateWithStartUpdateWorkflowInput],
         workflow_id: str,
     ) -> temporalio.api.workflowservice.v1.UpdateWorkflowExecutionRequest:
-        run_id = input.run_id if isinstance(input, StartWorkflowUpdateInput) else None
+        run_id, first_execution_run_id = (
+            (
+                input.run_id,
+                input.first_execution_run_id,
+            )
+            if isinstance(input, StartWorkflowUpdateInput)
+            else (None, None)
+        )
         req = temporalio.api.workflowservice.v1.UpdateWorkflowExecutionRequest(
             namespace=self._client.namespace,
             workflow_execution=temporalio.api.common.v1.WorkflowExecution(
                 workflow_id=workflow_id,
                 run_id=run_id or "",
             ),
-            first_execution_run_id=input.first_execution_run_id or "",
+            first_execution_run_id=first_execution_run_id or "",
             request=temporalio.api.update.v1.Request(
                 meta=temporalio.api.update.v1.Meta(
                     update_id=input.update_id or str(uuid.uuid4()),