Fix [#33962] Grease Pencil crashing blender

Own fault from grease pencil to curve enhacement. Fixed the bug itself, and made poll of convert operator more strict (no only active when there is something to convert, i.e. at least one stroke!).

git-svn-id: https://svn.blender.org/svnroot/bf-blender/trunk/blender@54000 954f8c5b-7b00-dc11-b283-0030488c597c