Allow undefined callbacks in Storage Observer (#3224)

Author: schmidt-sebastian

.changeset/popular-cups-melt.md

@@ -0,0 +1,7 @@
+---
+"firebase": patch
+"@firebase/storage-types": patch
+"@firebase/storage": patch
+---
+
+[fix] Updated the TypeScript types for all APIs using Observers to allow callback omission. 

packages/firebase/index.d.ts

@@ -7579,7 +7579,7 @@ declare namespace firebase.storage {
     on(
       event: firebase.storage.TaskEvent,
       nextOrObserver?:
-        | firebase.Observer<UploadTaskSnapshot>
+        | Partial<firebase.Observer<UploadTaskSnapshot>>
         | null
         | ((a: UploadTaskSnapshot) => any),
       error?: ((a: Error) => any) | null,

packages/storage-types/index.d.ts

@@ -91,7 +91,7 @@ export interface UploadTask {
   on(
     event: TaskEvent,
     nextOrObserver?:
-      | Observer<UploadTaskSnapshot>
+      | Partial<Observer<UploadTaskSnapshot>>
       | null
       | ((a: UploadTaskSnapshot) => any),
     error?: ((a: Error) => any) | null,