I think that those missing entries automatically gets a value of zero as the array is defined with a length (which means that the program sets a value for PT_NUM integers). So, it will not create an error as all elements get a value.
and null is defined to be zero.
It can be set one by one using platent[PT_TRANSTITANIUMELEMENT] = something;. Also, they will fill that up when the necessity arises (They know what they are doing). (Same as above)
It seems that enthalpy is used for the realistic heat simulation for temperature changes during phase transitions (Not sure, though).
Not sure of the context in TPT but enthalpy is the energy of something if I remember correctly (AS Chemistry).