Avro null default values bug

This is a fix for a null case in https://github.com/Parquet/parquet-mr/pull/299.

I also removed the shading of Jackson for Parquet-Avro (as discussed here: https://groups.google.com/forum/#!msg/parquet-dev/NVsG6q78b0M/lxSWMeVx3YsJ), since I hit the same issue when testing this change.