Formspecs: Close on metadata removal (#8348)

Formspecs will now close as soon the formspec string in the node metadata turns invalid.

src/client/gameui.cpp

@@ -302,3 +302,15 @@ void GameUI::toggleProfiler()
 		showTranslatedStatusText("Profiler hidden");
 	}
 }
+
+
+void GameUI::deleteFormspec()
+{
+	if (m_formspec)
+		m_formspec->quitMenu();
+
+	delete m_formspec;
+	m_formspec = nullptr;
+
+	m_formname.clear();
+}