Add abstract function for null information

[asfimport]

Reporter: Fiona La / @lafiona

_{Note: This issue was originally created as ARROW-12346. Please see the migration documentation for further details.}

[kevingurney]

Marking this as resolved.

A public Valid property, which represents the null values in an Arrow array, was added to the arrow.array.Array classes in #35655.

Example:

>> a = arrow.array(uint64([1, 2, 3]), Valid=[true, false, true])

a = 

[
  1,
  null,
  3
]

>> a.Valid

ans =

  3x1 logical array

   1
   0
   1