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:PageImages/Can%27t_find_constant_API_base

Same as Iceaf4e424cd929df7b3df46544844a4b0001f4d7.

Change-Id: I2c83ed1c508ac837536dd0d21d4938296ad6ba03