Validation condition in numeric question - not null

How can I create a validation condition stating that a numeric question MUST be completed? i.e. numeric question cannot be missing.
Something like:

!numericquestion.IsNull

Thank you for your help. This sounds very basic to me, but I could not find it in the forum or the documentation. I could add an enabling condition to all the following questions, but I hope that’s not the answer… Maybe there is a reasoning failure in my question, all ideas are welcome.