Fix NPE crash on NamedayDatabaseRefresher#loadSpecialNamedays()

[alexstyl]

Description

This PR fixes the following crash taken from Crashlytics:

Fatal Exception: java.lang.NullPointerException: Attempt to invoke interface method 'boolean android.database.Cursor.moveToNext()' on a null object reference
       at com.alexstyl.specialdates.events.NamedayDatabaseRefresher.loadSpecialNamedays(NamedayDatabaseRefresher.java:132)
       at com.alexstyl.specialdates.events.NamedayDatabaseRefresher.initialiseNamedaysIfEnabled(NamedayDatabaseRefresher.java:73)
       at com.alexstyl.specialdates.events.NamedayDatabaseRefresher.refreshNamedays(NamedayDatabaseRefresher.java:65)
       at com.alexstyl.specialdates.events.PeopleEventsUpdater.updateEventsIfSettingsChanged(PeopleEventsUpdater.java:64)
       at com.alexstyl.specialdates.events.PeopleEventsUpdater.updateEventsIfNeeded(PeopleEventsUpdater.java:45)
       at com.alexstyl.specialdates.events.PeopleEventsContentProvider.refreshEventsIfNeeded(PeopleEventsContentProvider.java:146)
       at com.alexstyl.specialdates.events.PeopleEventsContentProvider.query(PeopleEventsContentProvider.java:48)
       at android.content.ContentProvider.query(ContentProvider.java:978)
       at android.content.ContentProvider$Transport.query(ContentProvider.java:213)
       at android.content.ContentResolver.query(ContentResolver.java:502)
       at android.content.ContentResolver.query(ContentResolver.java:446)
       at com.alexstyl.specialdates.service.PeopleEventsProvider.getCelebrationDateFor(PeopleEventsProvider.java:55)
       at com.alexstyl.specialdates.service.DailyReminderIntentService.onHandleIntent(DailyReminderIntentService.java:63)
       at android.app.IntentService$ServiceHandler.handleMessage(IntentService.java:65)
       at android.os.Handler.dispatchMessage(Handler.java:102)
       at android.os.Looper.loop(Looper.java:135)
       at android.os.HandlerThread.run(HandlerThread.java:61)