SQL absolutely does have that problem. You have no way of enforcing or checking that an expression is supposed to be non-NULL; at best you get an error at runtime when you try to insert it into a given column, which is usually far too late.

This is only a problem insofar as any data constraint is a problem.

No, it's a specific problem of SQL that you can apply constraints to stored or quasistored columns but not to expressions. In a real typed language every expression has a type.