Rule check user_id exists if location_id is null using Eloquent or Query Builder

Question

Laravel 10 with MySQL. I have a table parallel_approvers.

# id, user_id, parallel_user_id, location_id, created_at, updated_at

I am trying to validate the following conditions: If the combination of user_id and parallel_user_id values with location_id exists, then it shouldn't allow any entry with the same user_id. In other words, I want to validate the scenario like this: If a user has a parallel user and its location_id is null, it will be applied for all users. Also, if all locations are defined, then there is no need to create a specific location.

$exists = \DB::table('parallel_approvers')
                ->where('user_id', $validated['user_id'])
                ->where('parallel_user_id', $validated['parallel_user_id'])
                ->whereNull('location_id')->exists();
            if ($exists) {
                $request->validate([
                    'user_id' => [
                        'required',
                        Rule::unique('parallel_approvers')
                            ->where(function ($query) use ($validated) {
                                $query->where('user_id', $validated['user_id'])
                                    ->where(function ($query) {
                                        $query->whereNotNull('location_id')
                                            ->orWhereNull('location_id')
                                            ->orWhereNotNull('parallel_user_id')
                                            ->orWhereNull('parallel_user_id');
                                    });
                            }),
                    ],
                ]);

                $parallelApprover = new ParallelApprover;

                if (!auth()->user()->hasRole('admin')) {
                    $parallelApprover->user_id = auth()->user()->id;
                } else {
                    $parallelApprover->user_id = $validated['user_id'];
                }

                $parallelApprover->parallel_user_id = $validated['parallel_user_id'];
                $parallelApprover->location_id = $validated['location_id'];
                $parallelApprover->created_at = now();
                $parallelApprover->updated_at = now();
                $parallelApprover->save();

                return Helper::sendResponse($parallelApprover, 'Success', 201);

            }