Locator: Guard settings saving against filters with empty ID

When constructing the settings key, an empty ID would lead to writing
(in this case removing) the whole Locator settings category, which
overwrites all indidvidual Locator settings. Don't save settings for
filters with empty ID at all.

Introduced when introducing clangd support to ClangCodeModel plugin.

Amends ecafdb7543

Fixes: QTCREATORBUG-26101
Change-Id: I4ecb4f86fa8a0ef2bc2417206629c9e322addb12
Reviewed-by: Christian Kandeler <christian.kandeler@qt.io>

src/plugins/coreplugin/locator/locator.cpp

@@ -318,7 +318,7 @@ void Locator::saveSettings() const
     s->remove(QString());
     s->setValue("RefreshInterval", refreshInterval());
     for (ILocatorFilter *filter : m_filters) {
-        if (!m_customFilters.contains(filter)) {
+        if (!m_customFilters.contains(filter) && filter->id().isValid()) {
             const QByteArray state = filter->saveState();
             s->setValueWithDefault(filter->id().toString(), state);
         }