[C#] ArrayData.NullCount should be a property

[asfimport]

ArrayData.NullCount should be a property so that it can be computed when necessary: for ex: after Slice(), NullCount is -1 and needs to be computed 

Reporter: Prashanth Govindarajan / @pgovind

_{Note: This issue was originally created as ARROW-5634. Please see the migration documentation for further details.}

[adamreeve]

@CurtHagenlocher I just came across this issue. We decided that we shouldn't change ArrayData.NullCount to be a property in #41144 to prevent breaking the ABI but instead trigger computing from the Array.NullCount property.

I think this can be closed.