Updated inv-mod to be consistent with existing u256-inv-mod impl. (#8584)

Author: orizi

corelib/src/math.cairo

@@ -89,7 +89,7 @@ pub fn inv_mod<
     a: NonZero<T>, n: NonZero<T>,
 ) -> Option<T> {
     if core::num::traits::One::<T>::is_one(@n.into()) {
-        return Some(core::num::traits::Zero::zero());
+        return None;
     }
     let (g, s, _, sub_direction) = egcd(a, n);
     if g.is_one() {

corelib/src/test/math_test.cairo

@@ -46,8 +46,8 @@ fn test_inv_mod() {
     assert(math::inv_mod(1, 24) == Some(1_u16), 'inv_mod(1, 24) != 1');
     assert(math::inv_mod(1, 5) == Some(1_u32), 'inv_mod(1, 5) != 1');
     assert(math::inv_mod(8, 24) == Option::<usize>::None, 'inv_mod(8, 24) != None');
-    assert(math::inv_mod(1, 1) == Some(0_usize), 'inv_mod(1, 1) != 0');
-    assert(math::inv_mod(7, 1) == Some(0_usize), 'inv_mod(7, 1) != 0');
+    assert_eq!(math::inv_mod(1, 1), Option::<usize>::None);
+    assert_eq!(math::inv_mod(7, 1), Option::<usize>::None);
 }
 
 #[test]