Add null to databaseAuthVariableOverride (#926)

Author: MathBunny

src/firebase-app.ts

@@ -47,7 +47,7 @@ export type AppHook = (event: string, app: FirebaseApp) => void;
  */
 export interface FirebaseAppOptions {
   credential?: Credential;
-  databaseAuthVariableOverride?: object;
+  databaseAuthVariableOverride?: object | null;
   databaseURL?: string;
   serviceAccountId?: string;
   storageBucket?: string;