docs(app-distribution): AppDistributionRelease.releaseNotes marked as nullable

matches upstream reality / future state firebase/firebase-ios-sdk#8602
related #5667

Author: mikehardy

packages/app-distribution/lib/index.d.ts

@@ -71,9 +71,9 @@ export namespace FirebaseAppDistributionTypes {
     buildVersion: string;
 
     /**
-     * The release notes for this build.
+     * The release notes for this build, possibly null if no release notes were provided.
      */
-    releaseNotes: string;
+    releaseNotes: string | null;
 
     /**
      * The URL for the build.