[6.x] Change MySql nullable modifier to allow generated columns to be not null

[DrCake]

This PR addresses this issue where it is impossible to make a MySql generated column be not null.
I ran into this issue when trying to run this migration:

class CreatePlacesTable extends Migration
{
    /**
     * Run the migrations.
     *
     * @return void
     */
    public function up()
    {
        Schema::create('places', function (Blueprint $table) {
            $table->bigIncrements('id');

            $table->string('name');
            $table->decimal('latitude', 8, 6);
            $table->decimal('longitude', 9, 6);
            $table->point('location', 4326)->storedAs('ST_SRID(Point(`longitude`, `latitude`), 4326)');
            $table->spatialIndex('location');

            $table->timestamps();
        });
    }

    /**
     * Reverse the migrations.
     *
     * @return void
     */
    public function down()
    {
        Schema::dropIfExists('places');
    }
}

which fails when trying to add the spatial index as it cannot be added to a 'nullable' column.

The change allows using the existing functionality of the nullable() method where you can pass false as the parameter to explictly mark the column as not null. This should hopefully limit the effect this change has on existing migrations by not changing the normal behaviour.

With this change, the migration would work by using this for the 'location' column

$table->point('location', 4326)->storedAs('ST_SRID(Point(`longitude`, `latitude`), 4326)')->nullable(false);

[taylorotwell]

Instead of this change, isn't the root of the problem the check above?

If that is unnecessary or unneeded, why are we not changing that instead?

[DrCake]

@staudenmeir mentioned in this issue #31398 that it's because we need to keep the old functionality to be able to support MariaDB.