[HBASE-7099][89-fb]i Bugfix in compaction hook if value is null

Author: adela

Summary: if value is null we would get nullpointer exception

Test Plan: ran TestLowerToUpperCompactionHook

Reviewers: aaiyer, manukranthk, rshroff

Reviewed By: aaiyer

CC: hbase-eng@

Differential Revision: https://phabricator.fb.com/D1191952

Task ID: 2095744

git-svn-id: https://svn.apache.org/repos/asf/hbase/branches/0.89-fb@1572708 13f79535-47bb-0310-9956-ffa450edef68