Improve load data behavior of SettingsStorage class

Previously it only handle the case of failed lookup, now it discard
invalid values when deserializing the database from disk.
Also checks whether the data is convertible to the intended type.

src/base/settingsstorage.cpp

@@ -295,7 +295,11 @@ QString TransactionalSettings::deserialize(const QString &name, QVariantHash &da
     // or that we don't touch directly in this code (eg disabled by ifdef). This ensures
     // that they will be copied over when save our settings to disk.
     for (const QString &key : asConst(settings->allKeys()))
-        data.insert(key, settings->value(key));
+    {
+        const QVariant value = settings->value(key);
+        if (value.isValid())
+            data[key] = value;
+    }
 
     return settings->fileName();
 }

src/base/settingvalue.h

@@ -48,12 +48,16 @@ public:
 
     T get(const T &defaultValue = {}) const
     {
-        if constexpr (std::is_enum_v<T>) {
+        if constexpr (std::is_enum_v<T>)
+        {
             const auto value = SettingsStorage::instance()->loadValue(m_keyName, {}).toString();
             return Utils::String::toEnum(value, defaultValue);
         }
-        else {
-            return SettingsStorage::instance()->loadValue(m_keyName, defaultValue).template value<T>();
+        else
+        {
+            const QVariant value = SettingsStorage::instance()->loadValue(m_keyName);
+            // check if retrieved value is convertible to T
+            return value.template canConvert<T>() ? value.template value<T>() : defaultValue;
         }
     }