correct nullability for generated columns

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.