#4537 Birth Cohort Pre-populator fails when p type code is 14

This pull request resolves the method is_p_type_15? from breaking when the p type local code is 14 in survey: INS_QUE_Birth_INT_M3.2_V3.1_PART_TWO 

participant.mother.participant would be nil and break when p_type was called on it

Redmine Reference: https://code.bioinformatics.northwestern.edu/issues/issues/show/4537