mark nil errors as nil instead of non-retryable (#4124) (#2618)

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

Author: modular-magician

.changelog/4124.txt

@@ -0,0 +1,3 @@
+```release-note:bug
+all: fixed misleading "empty non-retryable error" message that was appearing in debug logs
+```

google-beta/retry_transport.go

@@ -201,6 +201,9 @@ func (t *retryTransport) checkForRetryableError(resp *http.Response, respErr err
 		errToCheck = googleapi.CheckResponse(&respToCheck)
 	}
 
+	if errToCheck == nil {
+		return nil
+	}
 	if isRetryableError(errToCheck, t.retryPredicates...) {
 		return resource.RetryableError(errToCheck)
 	}