MySQL persistence: DimmerItem is a SwitchItem and needs to be checked prior to that

This prevents the following exception:

java.lang.IllegalArgumentException: No enum constant org.openhab.core.library.types.OnOffType.9
        at java.lang.Enum.valueOf(Enum.java:236)
        at org.openhab.core.library.types.OnOffType.valueOf(OnOffType.java:1)
        at org.openhab.persistence.mysql.internal.MysqlPersistenceService.query(MysqlPersistenceService.java:568)
        at org.openhab.core.persistence.internal.PersistenceManager.initialize(PersistenceManager.java:369)
        at org.openhab.core.persistence.internal.PersistenceManager.itemAdded(PersistenceManager.java:342)
        at org.openhab.core.persistence.internal.PersistenceManager.allItemsChanged(PersistenceManager.java:337)