[C++] Real null count for DictionaryArray

[R-JunmingChen]

Describe the enhancement requested

Currently, DictionaryArray::null_count() counts the number of nulls among the dict indices, but ignores nulls among dict values. It will be useful that we can count the same of the null number as the DictionaryArray is decoded. That would also help #37100. The implementation could be a member function like DictionaryArray::CountNullValues.

Component(s)

C++

[kou]

See also the related discussion: #37100 (comment)

We'll not change DictionaryArray::null_count() to keep backward compatibility.