How can I configure Seniority rule to return null when adjusted days is 0?

Summary:

If there is data in a Flexfield then "employment seniority date adjustment" ff should add 180 days in enterprise seniority date and return final adjusted seniority date. Incase the Flexfield is null then Seniority date should not return any date either Enterprise seniority date or default date defined in fast formula.

Content (please ensure you mask any confidential information):

DEFAULT_DATA_VALUE for PER_PERSON_EIT_ALL_PEI_INFORMATION_DATE1 is '1951/01/01' (DATE)
DEFAULT_DATA_VALUE for PER_PERSON_EIT_ALL_INFORMATION_TYPE is ' '
DEFAULT_DATA_VALUE for PER_SENDT_F_V3_SENIORITY_DATE is '1951/01/01' (DATE)