fix person_dob prepopulator #4018

Prepopulator trigger was not returning false when
Person.person_dob_date returned nil, as it should.
Further the test was wrong, where the test
description was contradicting the test assertion.(cherry picked from commit 7ecb783d32e502956f1128ccc41eda386a8d1f3a)
remove stubs
(cherry picked from commit e7e8ad9c8dd9d40666d1da64ca8e3a32466d4e4c)
Merge pull request #250 from NUBIC/4018_participant_verif_dob_pbs

4018 participant verif dob pbs