The lack of an NI number on an employee record will not prevent you from sending an FPS.
If you do have a new starter and no NI number make sure you complete the rest of the personal details accurately as HMRC will use these to confirm the NI number with yourselves.
If you include an employee on an FPS without an NI number completed this will automatically start the HMRC process for NI number verification. Once they confirm the identity of the employee they will send a NINO verification message to the registered contact for the PAYE scheme. This can then be saved in the payroll record.