Make should_receive return False when person does not exist
Make should_receive return False when person does not exist

We had a task group without without an assignee and this caused our
notifications code to break. This works because get_person_dict returns
{"email": "", "name": "", "id": -1} when the person undefined.

plamut/ggrc-core,AleksNeStu/ggrc-core,plamut/ggrc-core,AleksNeStu/ggrc-core,VinnieJohns/ggrc-core,plamut/ggrc-core,VinnieJohns/ggrc-core,AleksNeStu/ggrc-core,VinnieJohns/ggrc-core,selahssea/ggrc-core,selahssea/ggrc-core,selahssea/ggrc-core,plamut/ggrc-core,AleksNeStu/ggrc-core,VinnieJohns/ggrc-core,selahssea/ggrc-core