Add empty Sort Key support

[miguilimzero]

This pull request implements support for empty sort key definition. This kind of feature applies when the developer wants to set custom column store variables without setting a column as a sort key.

Example:

Schema::create('table', function (Blueprint $table) {
    $table->string('name');

    $table->sortKey()->with(['columnstore_segment_rows' => 100000]);
});

[miguilimzero]

@carlsverre I noticed the README still contains info about saying the package is a pre-release beta. Was the warning forgotten there or is it still valid? The package seems solid to me :)

[carlsverre]

This is awesome @srdante. Thanks for the PR!

And good call, I will start the process of marking it as "production ready".