Use defined() instead of constant()

To check for the existance of ApiBase::PARAM_HELP_MSG
while preventing warnings from being issued:
http://php.net/manual/en/function.constant.php#refsect1-function.constant-errors
https://www.mediawiki.org/wiki/Thread:Extension_talk:TextExtracts/PHp_Warnings_in_version_1.24

Fixes commit 9207edfa8157a402f7abcd7ba3256543fddd31d3

Change-Id: Iceaf4e424cd929df7b3df46544844a4b0001f4d7