[web] Fix shared albums (#1001)

This flow probably got broken when the select-all-for-a-day
functionality got added in https://github.com/ente-io/ente/pull/674
(haven't dug into this, I'm just guessing since that's where this all
got touched).

Thank you to @Bramas on our Discord for pointing this out, and also
providing the fix:
https://github.com/ente-io/ente/compare/main...Bramas:ente:fix-error-on-shared-album

Tested by
---------

Run normal web app on one terminal

NEXT_PUBLIC_ENTE_ENDPOINT=http://localhost:8080
NEXT_PUBLIC_ENTE_ALBUMS_ENDPOINT=http://localhost:3002 yarn dev

Run the albums listener on another port (need to do this since we're
running on localhost, and need to bind to a different origin):

NEXT_PUBLIC_ENTE_ENDPOINT=http://localhost:8080
NEXT_PUBLIC_ENTE_ALBUMS_ENDPOINT=http://localhost:3002 yarn dev:albums

Create a shared album, copy its public link, and verify that

- Before the fix we were getting an error (trying to access properties
of `galleryContext.selectedFile`)
- After the fix the shared album is visible

web/apps/photos/src/components/PhotoList/index.tsx

@@ -718,6 +718,9 @@ export function PhotoList({
     };
 
     useEffect(() => {
+        // Nothing to do here if nothing is selected.
+        if (!galleryContext.selectedFile) return;
+
         const notSelectedFiles = displayFiles?.filter(
             (item) => !galleryContext.selectedFile[item.id],
         );