[workflows] issue-write: Avoid race condition when PR branch is deleted (llvm#87118)

Fixes llvm#87102 .

Author: tstellar

.github/workflows/issue-write.yml

@@ -77,6 +77,15 @@ jobs:
             }
             const gql_result = await github.graphql(gql_query, gql_variables);
             console.log(gql_result);
+            // If the branch for the PR was deleted before this job has a chance
+            // to run, then the ref will be null.  This can happen if someone:
+            // 1. Rebase the PR, which triggers some workflow.
+            // 2. Immediately merges the PR and deletes the branch.
+            // 3. The workflow finishes and triggers this job.
+            if (!gql_result.repository.ref) {
+              console.log("Ref has been deleted");
+              return;
+            }
             console.log(gql_result.repository.ref.associatedPullRequests.nodes);
 
             var pr_number = 0;