Write out difficulty_invalid for last played diff

this fixes erratic crashes when saving the profile due to the last difficulty value somehow becoming null and reading garbage from memory

last played difficulty isn't relevant for the modern 4k meta and even if it were it would be less of a nuisance than crashing randomly, so while this is an inelegant workaround it's not functionally problematic

and hopefully it fixes all garbage difficulty enum crashes not just the single one we could reproduce

src/Profile.cpp

@@ -1528,7 +1528,7 @@ XNode* Profile::SaveEttGeneralDataCreateNode() const {
 	pGeneralDataNode->AppendChild("CharacterID", m_sCharacterID);
 	pGeneralDataNode->AppendChild("Guid", m_sGuid);
 	pGeneralDataNode->AppendChild("SortOrder", SortOrderToString(m_SortOrder));
-	pGeneralDataNode->AppendChild("LastDifficulty", DifficultyToString(m_LastDifficulty));
+	pGeneralDataNode->AppendChild("LastDifficulty", DifficultyToString(Difficulty_Invalid));
 	if (m_LastStepsType != StepsType_Invalid)
 		pGeneralDataNode->AppendChild("LastStepsType", GAMEMAN->GetStepsTypeInfo(m_LastStepsType).szName);
 	pGeneralDataNode->AppendChild(m_lastSong.CreateNode());