[MATLAB] Add `NumNulls` property to `arrow.array.Array` class

[sgilmore10]

Describe the enhancement requested

It would be nice if there was a NumNulls property on the arrow.array.Array base class. Currently, the only way to figure out the number of nulls is count the number of false values in the Valid array:

>> a = arrow.array([1 2 NaN 4 5 6 NaN 8 9 10 NaN]);
>> invalidValues = ~a.Valid;
>> numNulls = nnz(invalidValues)

numNulls =

     3

It would be nice if NumNulls was already a property on the array class. As @kou mentioned, we can use the arrow::Array::null_count() to get the number of nulls.

Component(s)

MATLAB

[kevingurney]

Adding good-first-issue and good-second-issue labels.

This would be a nice focused enhancement, and it might be a good way to learn about the C++ / libmexclass backend for the MATLAB Interface as a new contributor.

[sgilmore10]

take

[sgilmore10]

Issue resolved by pull request 47116
#47116