Hello Toni,
Maybe this stack overflow question can help you!?
It describes the concept of partial validation (with example) if you are using Java.
Another way might be to put the field on the form as hidden value (then it should not be null anymore) or try to "validate" with @Column(nullable=false).
If you save to a database this could work but will throw an exception if you try to save a null value at a later point.
I must admit that I haven't tested this so I could be totally wrong but the best solution is imho mentioned in the stack overflow link anyway.
Have a nice week-end
Henning