GlobalVarConfig shouldn't throw exceptions for null-valued config settings

Use array_key_exists rather than isset to test $GLOBALS.

Bug: 66986
Change-Id: I9beabd79e1d52966b9184c9647af5a2a31c19276
(cherry picked from commit 1a059dbbe2380e9ae752daf6c5583333b5bd5f07)