Update DrupalSetMessageLogger.php

Closes #3304922

src/Core/Service/StashLogger.php

@@ -26,7 +26,7 @@ class StashLogger extends AbstractLogger {
    * @param string $message
    * @param array $context
    */
-  public function log($level, $message, array $context = []) {
+  public function log($level, $message, array $context = []):void {
     $this->logs[] = [
       'level' => $level,
       'message' => $message,