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.