Remove nullables from IDataModels

[DDuarte]

The IDataModels in https://github.com/TrinityCore/WowPacketParser/tree/master/WowPacketParser/Store/Objects after 3ddbce572bddaf79c3d570cf406cc98d927acbad are using a lot of nullables which make the API a bit clumsy to use and in most cases the null fields do not make any sense, e.g, what does it mean having a CreatureTemplate object with Entry set to null?

@Carbenium can give more info on this

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