[11.x] Fix attribute mutator access in loadMissing

[SychO9]

Issue
When using ->loadMissing collection method on a nested relationship it can cause all the models in the collection to be converted to JSON which causes attribute mutators to be accessed. This is normally not a problem, unless the mutators need to access other relationships, in which case it can become an issue as certain relationships can get loaded unnecessarily.

Proposed Solution
The cause is the use of ->whereNotNull collection method, which leads to the JSON conversion. The PR changes that bit to a simpler ->filter access.

Additional Context
Backtrace when using ->whereNotNull

array:105 [▼
  ...app code  

  8 => array:3 [▼
    "file" => "/home/.../vendor/illuminate/database/Eloquent/Concerns/HasAttributes.php"
    "line" => 689
    "function" => "getContentAttribute"
  ]
  9 => array:3 [▼
    "file" => "/home/.../vendor/illuminate/database/Eloquent/Concerns/HasAttributes.php"
    "line" => 739
    "function" => "mutateAttribute"
  ]
  10 => array:3 [▼
    "file" => "/home/.../vendor/illuminate/database/Eloquent/Concerns/HasAttributes.php"
    "line" => 277
    "function" => "mutateAttributeForArray"
  ]
  11 => array:3 [▼
    "file" => "/home/.../vendor/illuminate/database/Eloquent/Concerns/HasAttributes.php"
    "line" => 215
    "function" => "addMutatedAttributesToArray"
  ]
  12 => array:3 [▼
    "file" => "/home/.../vendor/illuminate/database/Eloquent/Model.php"
    "line" => 1664
    "function" => "attributesToArray"
  ]
  13 => array:3 [▼
    "file" => null
    "line" => null
    "function" => "Illuminate\Database\Eloquent\{closure}"
  ]
  14 => array:3 [▼
    "file" => "/home/.../vendor/illuminate/database/Eloquent/Concerns/PreventsCircularRecursion.php"
    "line" => 46
    "function" => "call_user_func"
  ]
  15 => array:3 [▼
    "file" => "/home/.../vendor/illuminate/database/Eloquent/Model.php"
    "line" => 1663
    "function" => "withoutRecursion"
  ]
  16 => array:3 [▼
    "file" => "/home/.../vendor/illuminate/database/Eloquent/Model.php"
    "line" => 1695
    "function" => "toArray"
  ]
  17 => array:3 [▼
    "file" => "/home/.../vendor/illuminate/database/Eloquent/Model.php"
    "line" => 1680
    "function" => "jsonSerialize"
  ]
  18 => array:3 [▼
    "file" => "/home/.../vendor/illuminate/database/Eloquent/Model.php"
    "line" => 2391
    "function" => "toJson"
  ]
  19 => array:3 [▼
    "file" => "/home/.../vendor/illuminate/support/helpers.php"
    "line" => 68
    "function" => "__toString"
  ]
  20 => array:3 [▼
    "file" => "/home/.../vendor/illuminate/support/helpers.php"
    "line" => 166
    "function" => "blank"
  ]
  21 => array:3 [▼
    "file" => "/home/.../vendor/illuminate/support/helpers.php"
    "line" => 483
    "function" => "filled"
  ]
  22 => array:3 [▼
    "file" => "/home/.../vendor/illuminate/collections/functions.php"
    "line" => 20
    "function" => "transform"
  ]
  23 => array:3 [▼
    "file" => "/home/.../vendor/illuminate/collections/Traits/EnumeratesValues.php"
    "line" => 1097
    "function" => "Illuminate\Support\enum_value"
  ]
  24 => array:3 [▼
    "file" => null
    "line" => null
    "function" => "Illuminate\Support\Traits\{closure}"
  ]
  25 => array:3 [▼
    "file" => "/home/.../vendor/illuminate/collections/Arr.php"
    "line" => 930
    "function" => "array_filter"
  ]
  26 => array:3 [▼
    "file" => "/home/.../vendor/illuminate/collections/Collection.php"
    "line" => 395
    "function" => "where"
  ]
  27 => array:3 [▼
    "file" => "/home/.../vendor/illuminate/collections/Traits/EnumeratesValues.php"
    "line" => 623
    "function" => "filter"
  ]
  28 => array:3 [▼
    "file" => "/home/.../vendor/illuminate/collections/Traits/EnumeratesValues.php"
    "line" => 645
    "function" => "where"
  ]
  29 => array:3 [▼
    "file" => "/home/.../vendor/illuminate/database/Eloquent/Collection.php"
    "line" => 274
    "function" => "whereNotNull"
  ]
  30 => array:3 [▼
    "file" => "/home/.../vendor/illuminate/database/Eloquent/Collection.php"
    "line" => 245
    "function" => "loadMissingRelation"
  ]
  31 => array:3 [▼
    "file" => "/home/.../vendor/flarum/core/src/Api/Endpoint/Concerns/HasEagerLoading.php"
    "line" => 128
    "function" => "loadMissing"
  ],
  
  ...app code
];

[mortenscheel]

@SychO9 thanks! This issue very nearly drove me insane.