[7.x] Reset select bindings when setting select

[sebsel]

fixes #32526

---

This PR resets the select bindings when the select clause is set to something else.

Most operations on the Query Builder are additive: when you have an instance of it, and you call ->where('column', 'value') on it, the clause is added to the underlying query.

This also works with sub-queries: bindings are merged into the query. Take for example this orderBy():

User::orderBy(
    Book::whereColumn('users.id', 'user_id')
        ->where('has_valid_isbn', true)
        ->select(DB::raw('count(*)'))
)->orderBy('created_at');

// select * from "users" 
// order by
//     (select count(*) from "books" where "users"."id" = "user_id" and "has_valid_isbn" = ?) asc, 
//     "created_at" asc"

// Bindings: [true]

Setting a select query, however, overwrites previous selects. (Use addSelect() when you want to add.)

$builder = Book::select('title');
$builder->toSql(); //  select "title" from "books"
$builder->select('*');
$builder->toSql(); //  select * from "books"
$builder->addSelect('title');
$builder->toSql(); //  select *, "title" from "books"

When using a subquery in the select clause, registered bindings would currently stay in the query. MySQL 8.0 seems to be fine with that, but Postgres gives an error when this happens.

>>> $query = App\User::withCount('books');
=> Illuminate\Database\Eloquent\Builder {#3013}
>>> $query->toSql();
=> "select "users".*, (select count(*) from "books" where "users"."id" = "books"."user_id" and "has_valid_isbn" = ?) as "books_count" from "users""
>>> $query->getBindings();
=> [
     true,
   ]
>>> $query->select('*')->toSql();
=> "select * from "users""
>>> $query->select('*')->getBindings();
=> [
     true,
   ]
>>> $query->select('*')->get();
Illuminate/Database/QueryException with message 'SQLSTATE[08P01]: <<Unknown error>>: 7 ERROR:  bind message supplies 1 parameters, but prepared statement "pdo_stmt_00000003" requires 0 (SQL: select * from "users")

This PR fixes that by setting the bindings for the select part of the queries to an empty array when an overwriting ->select() call is made.

[driesvints]

@sebsel best that you explain why this is needed (see the pr template)

[sebsel]

@driesvints I updated the text. Hope this works, because I can't seem to find a PR template.

[driesvints]

@sebsel This is visible whenever you open up a new PR:

https://raw.githubusercontent.com/laravel/framework/7.x/.github/PULL_REQUEST_TEMPLATE.md