NotesService: Add unit tests

Fixed toUserDto method of UsersService: If a user has no email an empty string should be returned (like with the photoUrl) instead of null.

Signed-off-by: Philip Molares <philip.molares@udo.edu>

src/users/users.service.ts

@@ -63,7 +63,7 @@ export class UsersService {
       userName: user.userName,
       displayName: user.displayName,
       photo: this.getPhotoUrl(user),
-      email: user.email,
+      email: user.email ?? '',
     };
   }
 }