Fix down() for MakeSettingsValueNullable migration.

[likemusic]

If settings already contains null values, they should be converted to empty strings before altering table.

To prevent:

   Illuminate\Database\QueryException  : SQLSTATE[22004]: Null value not allowed: 1138 Invalid use of NULL value (SQL: ALTER TABLE settings CHANGE value value TEXT CHARACTER SET utf8mb4 NOT NULL COLLATE `utf8mb4_unicode_ci`)

  at /var/www/vendor/laravel/framework/src/Illuminate/Database/Connection.php:669
    665|         // If an exception occurs when attempting to run a query, we'll format the error
    666|         // message to include the bindings with SQL, which will make this exception a
    667|         // lot more helpful to the developer instead of just the database's errors.
    668|         catch (Exception $e) {
  > 669|             throw new QueryException(
    670|                 $query, $this->prepareBindings($bindings), $e
    671|             );
    672|         }
    673|

  Exception trace:

  1   Doctrine\DBAL\Driver\PDOException::("SQLSTATE[22004]: Null value not allowed: 1138 Invalid use of NULL value")
      /var/www/vendor/doctrine/dbal/lib/Doctrine/DBAL/Driver/PDOStatement.php:123

  2   PDOException::("SQLSTATE[22004]: Null value not allowed: 1138 Invalid use of NULL value")
      /var/www/vendor/doctrine/dbal/lib/Doctrine/DBAL/Driver/PDOStatement.php:121

  Please use the argument -v to see more details.

[codecov]

Codecov Report

Merging #4838 into 1.4 will not change coverage.
The diff coverage is n/a.

@@            Coverage Diff            @@
##                1.4    #4838   +/-   ##
=========================================
  Coverage     62.93%   62.93%           
  Complexity     1373     1373           
=========================================
  Files           194      194           
  Lines          3996     3996           
=========================================
  Hits           2515     2515           
  Misses         1481     1481           

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