return nil instead of op, which is an interface containing nil

Signed-off-by: Modular Magician <[email protected]>

google/common_operation.go

@@ -117,7 +117,7 @@ func CommonRefreshFunc(w Waiter) resource.StateRefreshFunc {
 			for _, e := range getAllTypes(err, &googleapi.Error{}, &url.Error{}) {
 				if isRetryableError(e, predicates) {
 					log.Printf("[DEBUG] Dismissed error on GET of operation '%v' retryable: %s", w.OpName(), err)
-					return op, "done: false", nil
+					return nil, "done: false", nil
 				}
 			}