typing: refactor type adaptations in AEqExpr and ANeExpr

Signed-off-by: Jean Privat <jean@pryen.org>
typing: new warning useless-null-test that detect == and != between non-nullables and null

Signed-off-by: Jean Privat <jean@pryen.org>
src: remove useless comparisons on null

Signed-off-by: Jean Privat <jean@pryen.org>
lib: remove useless comparison on null that broke tests

Signed-off-by: Jean Privat <jean@pryen.org>
tests: update some tests related to useless-null-test

Signed-off-by: Jean Privat <jean@pryen.org>