check owner is not zero on initializer and improve comments

packages/access/src/ownable/ownable.cairo

@@ -136,6 +136,8 @@ pub mod OwnableComponent {
 
         /// Starts the two-step ownership transfer process by setting the pending owner.
         ///
+        /// Setting `newOwner` to the zero address is allowed; this can be used to cancel an
+        /// initiated ownership transfer.
         /// Requirements:
         ///
         /// - The caller is the contract owner.
@@ -278,8 +280,13 @@ pub mod OwnableComponent {
     > of InternalTrait<TContractState> {
         /// Sets the contract's initial owner.
         ///
+        /// Requirements:
+        ///
+        /// - `owner` is not the zero address.
+        ///
         /// This function should be called at construction time.
         fn initializer(ref self: ComponentState<TContractState>, owner: ContractAddress) {
+            assert(!owner.is_zero(), Errors::ZERO_ADDRESS_OWNER);
             self._transfer_ownership(owner);
         }
 

packages/access/src/tests/test_ownable.cairo

@@ -44,6 +44,13 @@ fn test_initializer_owner() {
     assert_eq!(new_owner, OWNER());
 }
 
+#[test]
+#[should_panic(expected: ('New owner is the zero address',))]
+fn test_initializer_zero_owner() {
+    let mut state = COMPONENT_STATE();
+    state.initializer(ZERO());
+}
+
 //
 // assert_only_owner
 //