error message on the admin/structure/context page when no dart tags exist

In that case, this is the error:
Notice: Undefined variable: options in dart_context_reaction_tags->options_form() (line 28 of sites/all/modules/contrib/dart/plugins/contexts/dart_context_reaction_tags.inc).

Attached is a quick patch that removes the error an gives a bit of useful information to the user. As long as testbot like it, Ill commit it