[HBASE-11187] Fix null pointer.

Author: elliott

Summary: Fix null pointer when there are no outstanding requests.

Test Plan: mvn test -Dtest=TestEmptyMetaInfo

Reviewers: manukranthk

Reviewed By: manukranthk

Subscribers: hbase-eng@

Differential Revision: https://phabricator.fb.com/D1336725

git-svn-id: https://svn.apache.org/repos/asf/hbase/branches/0.89-fb@1596348 13f79535-47bb-0310-9956-ffa450edef68