[5.8] Fix collections with JsonSerializable items and mixed values

[staudenmeir]

#11739 added support for collections with JsonSerializable items.

It only considered cases where jsonSerialize() returns an array, but the method can return mixed values.

If it does, you end up with an invalid collection where $items is not an array:

collect(new Carbon);

// expected
Collection { ▼
  #items: array:1 [▼
    0 => "2019-07-17T16:24:29.660974Z"
  ]
}

// actual
Collection { ▼
  #items: "2019-07-17T16:24:29.660974Z"
}

Fixes #29187.