fix(emails): visibility can be null (#756)

According to the [`/user/emails` schema](https://docs.github.com/en/rest/users/emails?apiVersion=2022-11-28#list-email-addresses-for-the-authenticated-user),
the `visibility` field in the response can be `null`. The
`UserEmailInfo` made it mandatory, causing deserialization failures
during requests against real GitHub APIs like this:

```
invalid type: null, expected string or map
```

Author: dndx

src/models.rs

@@ -1106,7 +1106,7 @@ pub struct UserEmailInfo {
     pub email: String,
     pub primary: bool,
     pub verified: bool,
-    pub visibility: EmailVisibilityState,
+    pub visibility: Option<EmailVisibilityState>,
 }
 
 #[derive(Debug, Clone, Serialize, Deserialize)]

tests/resources/user_emails.json

@@ -4,5 +4,11 @@
     "primary": true,
     "verified": true,
     "visibility": "private"
+  },
+  {
+    "email": "[email protected]",
+    "primary": true,
+    "verified": true,
+    "visibility": null
   }
 ]

tests/user_emails_tests.rs

@@ -54,7 +54,7 @@ async fn should_respond_to_primary_email_visibility() {
     );
     let response = result.unwrap();
     let visibility = response.first().unwrap().visibility;
-    assert_eq!(visibility, EmailVisibilityState::Private);
+    assert_eq!(visibility, Some(EmailVisibilityState::Private));
 }
 
 #[tokio::test]
@@ -78,7 +78,10 @@ async fn should_respond_to_email_list() {
     );
     let response = result.unwrap();
     let visibility = response.items.first().unwrap().visibility;
-    assert_eq!(visibility, EmailVisibilityState::Private);
+    assert_eq!(visibility, Some(EmailVisibilityState::Private));
+
+    let visibility = response.items[1].visibility;
+    assert_eq!(visibility, None);
 }
 
 #[tokio::test]
@@ -102,7 +105,7 @@ async fn should_respond_to_public_email_list() {
     );
     let response = result.unwrap();
     let visibility = response.items.first().unwrap().visibility;
-    assert_eq!(visibility, EmailVisibilityState::Private);
+    assert_eq!(visibility, Some(EmailVisibilityState::Private));
 }
 
 #[tokio::test]