fix: fix error handling for empty HTTP responses

- Change the condition to check for `http.StatusNoContent` instead of `http.StatusOK` before returning an error.

Signed-off-by: appleboy <appleboy.tw@gmail.com>

plugin.go

@@ -283,7 +283,7 @@ func (p *Plugin) SendMessage(ctx context.Context) error {
 	}
 	defer resp.Body.Close()
 
-	if resp.StatusCode != http.StatusOK {
+	if resp.StatusCode != http.StatusNoContent {
 		return fmt.Errorf("failed to send message, status code: %d", resp.StatusCode)
 	}