Merge branch '6.4' into 7.0

* 6.4:
  Fix implicit nullable parameters
  Remove whitespaces from block form_help
  Fix TypeError on ProgressBar

Helper/ProgressBar.php

@@ -183,9 +183,9 @@ public function setMessage(string $message, string $name = 'message'): void
         $this->messages[$name] = $message;
     }
 
-    public function getMessage(string $name = 'message'): string
+    public function getMessage(string $name = 'message'): ?string
     {
-        return $this->messages[$name];
+        return $this->messages[$name] ?? null;
     }
 
     public function getStartTime(): int