Currently all generated columns are created nullable. Because we don't know whether an expression may be NULL or not until we create an item tree and fix_field it. And this only t happens after the .frm is written.

A way to workaround it is to create all generated fields nullable, as now. But after the .frm is opened and fields are fixes — check maybe_null property of all generated columns and mark them not nullable, as appropriate (set null_ptr=0, etc). By doing that we'll lose one bit in the null bitmap per generated not nullable field.