Laravel Collection where key/value equal

I have a collection that I am passing to my view. There are 3 rows:

Collection {#869 ▼
#items: array:3 [▼
0 => Customer {#868 ▼
  #table: "customers"
  #guarded: []
  #connection: "mysql"
  #primaryKey: "id"
  #keyType: "int"
  +incrementing: true
  #with: []
  #withCount: []
  #perPage: 15
  +exists: true
  +wasRecentlyCreated: false
  #attributes: array:10 [▼
    "id" => 17
    "business_id" => 1
    "first_name" => "Bob"
    "last_name" => null
    "email" => null
    "phone" => null
    "sent_at" => null
    "created_at" => "2017-10-03 16:18:30"
    "updated_at" => "2017-10-03 16:18:30"
    "deleted_at" => null
  ]
  #original: array:10 [▶]
  #changes: []
  #casts: []
  #dates: []
  #dateFormat: null
  #appends: []
  #dispatchesEvents: []
  #observables: []
  #relations: []
  #touches: []
  +timestamps: true
  #hidden: []
  #visible: []
  #fillable: []
}
1 => Customer {#867 ▶}
2 => Customer {#866 ▶}
]
}

Out of these 3 customers 1 of them has a "sent_at" column that is null that I want to count within my view. I understand you can do this in the controller but I want to modify the collection in the view.

I have tried this in my view and it returns no results:

$customers_pending = $customers_all->where('sent_at', null);
$customers_pending->count();
dd($customers_pending);

I guess I'm not fully understanding collections and modifying them. $customers_pending should return = 1

via Chebli Mohamed