Handle deleted labels

Author: ehuss

Diff for: src/github.rs

@@ -1168,7 +1168,9 @@ pub enum IssuesAction {
     },
     Unlabeled {
         /// The label removed from the issue
-        label: Label,
+        ///
+        /// The `label` is `None` when a label is deleted from the repository.
+        label: Option<Label>,
     },
     Locked,
     Unlocked,

Diff for: src/handlers/notify_zulip.rs

@@ -32,7 +32,7 @@ pub(super) async fn parse_input(
     };
 
     match &event.action {
-        IssuesAction::Labeled { label } | IssuesAction::Unlabeled { label } => {
+        IssuesAction::Labeled { label } | IssuesAction::Unlabeled { label: Some(label) } => {
             let applied_label = label.clone();
             Ok(config
                 .labels