LanguageClient: avoid crash on collecting clients for a null setting

And assert in that case.

Change-Id: I7b74589e6f08eb57e1b08bf509729bef32d0a9ff
Reviewed-by: Marcus Tillmanns <marcus.tillmanns@qt.io>

src/plugins/languageclient/languageclientmanager.cpp

@@ -387,6 +387,7 @@ void LanguageClientManager::enableClientSettings(const QString &settingsId, bool
 QList<Client *> LanguageClientManager::clientsForSetting(const BaseSettings *setting)
 {
     QTC_ASSERT(managerInstance, return {});
+    QTC_ASSERT(setting, return {});
     auto instance = managerInstance;
     return instance->m_clientsForSetting.value(setting->m_id);
 }