[OoT] Handle empty "extra cutscenes" entries better

[Dragorn421]

In oot scene object properties, Cutscenes tab, there is a feature to select some "extra cutscenes" to export with the scene but not attached to any scene header.

When there is an entry for one of those but no cutscene object is picked, fast64 gives a 'non-specific "None has no attribute blabla" error instead of a better error or just ignoring the entry

Fast64 should just ignore an entry if it has no cutscene object, sounds harmless

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See HM https://discord.com/channels/388361645073629187/451783162859749386/1259243920093745252