[thrift] return empty string instead of NULL

Summary: Turns out they're different, oops
Reviewed By: marcel
Test Plan: recompile, reinstall, synapse:tablet.get() on a nonexistent cell (which returns an empty string)
Revert: ok


git-svn-id: https://svn.apache.org/repos/asf/incubator/thrift/trunk@665318 13f79535-47bb-0310-9956-ffa450edef68
1 file changed