[C++][Parquet] Fix WriterBatch API to handle NULL values

[asfimport]

apache/parquet-cpp#157
introduced batching of values WriteMiniBatch of being written. The implementation missed handling cases where input data has NULL values.

Reporter: Deepak Majeti / @majetideepak
Assignee: Deepak Majeti / @majetideepak

_{Note: This issue was originally created as PARQUET-719. Please see the migration documentation for further details.}

[asfimport]

Deepak Majeti / @majetideepak:
PR: apache/parquet-cpp#160

[asfimport]

Deepak Majeti / @majetideepak:
Issue resolved by pull request 160
apache/parquet-cpp#160