Support for null in functions

[mnbjhu]

Currently functions only support the NotNull type.
Most functions in neo4j have the property f(null) = null and are null otherwise.
For this reason I think it makes sense to change most of the functions to have two signatures (probably implemented as an object with two 'invoke' functions)
These would be:
(A) -> B
(A?) -> B?
An example would be adding two longs, the function signatures would be:
operator fun LongReturn.plus(other: LongReturn): LongReturn // Current
operator fun Nullable<Long, LongReturn>.plus(other: LongReturn): Nullable<Long, LongReturn>
operator fun LongReturn.plus(other: Nullable<Long, LongReturn>): Nullable<Long, LongReturn>
operator fun Nullable<Long, LongReturn>.plus(other: Nullable<Long, LongReturn>): Nullable<Long, LongReturn>

I think this would both let the user take advantage of null safety when not using null types as well has not slowing people down by haivng to handle null types