PARQUET-19: Fix NPE when an empty file is included in a Hive query that uses CombineHiveInputFormat

Make sure the valueObj instance variable is always initialized.  This change is neeeded when running a Hive query that uses the CombineHiveInputFormat and the first file in the combined split is empty.  This can lead to a NullPointerException because the valueObj is null when the CombineHiveInputFormat calls the createValue method.

Author: Matthieu Martin <ma.tt.b.ma.rt.in+parquet@gmail.com>

Closes #19 from matt-martin/fix_for_empty_files_NPE_with_CombineHiveInputFormat and squashes the following commits:

6c3a7f5 [Matthieu Martin] Make sure the valueObj instance variable is always initialized.  This change is neeeded when running a Hive query that uses the CombineHiveInputFormat and the first file in the combined split is empty.  This can lead to a NullPointerException because the valueObj is null when the CombineHiveInputFormat calls the createValue method.