You are not supposed to leave any cell blank in any form. If you leave it blank and sign the form, and later on someone fills the cell with their name, then you cannot challenge it (or at least you will be dragged into an unnecessary legal battle) that you had left the cell blank and there was some illegal interference. Therefore, you may ask them to fill in the name of their immediate family members, and when they acquire a family, let them change the nomination by submitting another nomination form. Normally, when an unmarried person joins, they would nominate their father or mother.
In the Pension part, they will not write anybody as family members but will write the name of their father/mother as persons eligible to receive the benefits in case they die without leaving a family. After marriage, they will submit another Form 2 with the name of the spouse as their nominee under PF and pension schemes. Also, please note that under the Pension scheme, only the spouse and children are family members who are eligible to receive a family pension in the event of the member's death. Under the Pension scheme, there is a nomination only if the member has no family. That is why the PF form (Form 2) is a Nomination cum Declaration form, i.e., Nomination under the PF scheme and Declaration of family members or nomination if there is no family under the Pension Scheme.
Regards.