I've not heard of this option whereIfIsNotEmpty, but if category is null (empty) you could use whereNotNull('category') or maybe where('category', '<>', '')

Make use of conditional clauses: https://laravel.com/docs/5.4/queries#conditional-clauses

This should solve your problem

$data = $request->all();

$product =  Products::select('id', 'title)
->when(!empty($data['category']) , function ($query) use($data){
return $query->where('category',$data['category']);
})
->when (!empty($data['title']) , function ($query) use($data){
return $query->where('title',$data['title']);
})
->get()


return $product ;

You should use conditional clause. Here take a look at this piece of documentation.

https://laravel.com/docs/5.4/queries#conditional-clauses