fix(auth): only use currentUser as default if signed in

Auth.currentUser is `null` during initialization. `useAuthState` therefore
returned `null` while firebase was still fetching the current state.
With this change, `useAuthState` never returns a wrong state while
firebase is still loading.

src/auth/useAuthState.spec.ts

@@ -32,8 +32,8 @@ describe("initial state", () => {
         expect(result.current).toStrictEqual([currentUser, false, undefined]);
     });
 
-    it("should return undefined when currentUser is undefined", () => {
-        const mockAuth = {} as Auth;
+    it("should return undefined when currentUser is null", () => {
+        const mockAuth = { currentUser: null } as Auth;
 
         onAuthStateChangedMock.mockImplementation(() => () => {});
 

src/auth/useAuthState.ts

@@ -20,5 +20,5 @@ export function useAuthState(auth: Auth): UseAuthStateResult {
         []
     );
 
-    return useListen(auth, onChange, () => true, auth.currentUser);
+    return useListen(auth, onChange, () => true, auth.currentUser ?? undefined);
 }