[11.x] add nullOnUpdate() method to ForeignKeyDefinition

[gisuNasr]

Summary:

This PR introduces the nullOnUpdate() method to the ForeignKeyDefinition class. This method allows for setting the foreign key value to null when the referenced record is updated.

Usage:

$table->foreign('user_id')
      ->references('id')
      ->on('users')
      ->nullOnUpdate();