sql: fix fk set null clause

After changing behavior of the `IS` operator (#b3a3ddb5),
`SET NULL` was rewritten to use `EQ` instead. Which doesn't respect
NULLs.

This commit fixes the null related behavior by emitting logical
constructions equivalent for this case to old `IS`.
The new expression works differently than old `IS` for nulls, however
the difference doesn't change anything, because matched rows are then
searched in a child table with `EQ` expression which do not match nulls.
Before:
`oldval` old_is `newval`
Now:
`oldval` is_null or (`newval` is_not_null and `oldval` eq `newval`)

Closes #3645