alt attribute should default to null in Media model

[selfsimilar]

The recently added alt column's default empty string value means that new records will always include the alt column as a value to insert (e.g.

INSERT INTO `media`
(`alt`, `disk`, `filename`, `extension`, `mime_type`, `updated_at`, `created_at`)
values
('', 'public', 'example', 'png', 'image/png', '2024-01-01 00:00:00',  '2024-01-01 00:00:00')

In addition to creating more verbose queries and slightly larger records on disk, this will break when using Spatie's Laravel Translatable package if, as is recommended, the column type is changed to JSON which still stores the value as LONGTEXT but adds a constraint of json_valid(alt) which will accept null but not an empty string.