Fix null work-priority cache for generated pawns

[scarf005]

What

fix Select Attached Mechanoid mod not working with defaults mod

Reproduction

1. enable biotech DLC, defaults, and Select Attached Mechanoid mod
2. install a mechlink and choose a starting mechanoid
3. selected mechanoid fails to spawn with following error message

Exception running QuestNode_Root_MechanitorStartingMech: System.NullReferenceException
  at Defaults.WorkPriorities.CompatibilityPatch_Pawn.Postfix
  at Verse.Pawn.WorkTypeIsDisabled
  at RimWorld.Pawn_WorkSettings.EnableAndInitialize
  at SelectAttachedMechanoid.StartingMechSelection.RunSelectedStartingMechQuest

CompatibilityPatch_Pawn calls Contains on cachedDisabledWorkTypes, which can be null while a newly generated mechanoid initializes work settings

Fix

added null check. tested locally and was able to choose and spawn mechanoid again