[thrift] Fix extension deserialization call with no classname

Summary: The extension expects a classname as a string and does not allow null;
  use the empty string instead of null in the absence of a classname to pass.

Reviewed By: mcslee
Test Plan: php -l
Revert: svn


git-svn-id: https://svn.apache.org/repos/asf/incubator/thrift/trunk@665347 13f79535-47bb-0310-9956-ffa450edef68
1 file changed