Implement Merge test cases (#32)

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Author: MattyTheHacker

src/autoupdater.ts

@@ -231,7 +231,7 @@ export class AutoUpdater {
       head: baseRef,
     };
 
-    if (mergeMsg !== null && mergeMsg.length > 0) {
+    if (typeof mergeMsg === 'string' && mergeMsg.length > 0) {
       mergeOpts.commit_message = mergeMsg;
     }