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ARROW-12513: [C++][Parquet] Parquet Writer always puts null_count=0 in Parquet statistics for dictionary-encoded array with nulls

This fixes two issues.
* The null_count must be obtained from the indices array and not the values array
* The min/max should be based on referenced values and not all values in the values array

Note: This further adds a dependency from parquet onto arrow::compute (I use it both to compute the unique indices and to take the referenced values).  This dependency already existed (column_writer.cc relies on arrow::compute::Cast) so I'm pretty sure this isn't a problem.  Related: ARROW-8891

Closes #10729 from westonpace/bugfix/ARROW-12513--c-parquet-parquet-writer-always-puts-null_cou

Authored-by: Weston Pace <weston.pace@gmail.com>
Signed-off-by: Weston Pace <weston.pace@gmail.com>