Fix error fetching merge queue when no merge queue (microsoft#5487)

Author: alexr00

src/github/githubRepository.ts

@@ -456,7 +456,7 @@ export class GitHubRepository implements vscode.Disposable {
 			});
 
 			Logger.debug('Fetch branch has merge queue - done', GitHubRepository.ID);
-			const mergeMethod = parseMergeMethod(result.data.repository.mergeQueue.configuration?.mergeMethod);
+			const mergeMethod = parseMergeMethod(result.data.repository.mergeQueue?.configuration?.mergeMethod);
 			if (mergeMethod) {
 				this._branchHasMergeQueue.set(branch, mergeMethod);
 			}

src/github/graphql.ts

@@ -399,8 +399,8 @@ export interface MarkPullRequestReadyForReviewResponse {
 
 export interface MergeQueueForBranchResponse {
 	repository: {
-		mergeQueue: {
-			configuration: {
+		mergeQueue?: {
+			configuration?: {
 				mergeMethod: MergeMethod;
 			}
 		}