Fixed GetAbilitiesList would fire null and throw errors

Noticed sometimes that AbilityBook.GetAbilitiesList(Spell.CodeEnumSpellTag.Mount) would return a null table, changed the if statement which should prevent this error from happening. First time coding in LUA so I'm not 100% sure it will fix the problem though i haven't seen it happen since.

Sorry posted my first one to master. https://github.com/vikinghug/VikingActionBarFrame/pull/21