migrations:diff always creates Version file

[derlarsen]

Using the following command

./doctrine migrations:diff

does always create a version file even if there are no changes to be made to the current schema.

The content of this file is for example

namespace Application\Migrations;

use Doctrine\DBAL\Migrations\AbstractMigration,
    Doctrine\DBAL\Schema\Schema;

/**
 * Auto-generated Migration: Please modify to your need!
 */
class Version20121210110653 extends AbstractMigration
{
    public function up(Schema $schema)
    {
        // this up() migration is autogenerated, please modify it to your needs
        $this->abortIf($this->connection->getDatabasePlatform()->getName() != "mysql");

    }

    public function down(Schema $schema)
    {
        // this down() migration is autogenerated, please modify it to your needs
        $this->abortIf($this->connection->getDatabasePlatform()->getName() != "mysql");

    }
}

In Doctrine\DBAL\Migrations\Tools\Console\Command\DiffCommand there is always the 'abortIf()' line added to the Version-Code.

As we want to use Migrations in a semi automatic build process these 'empty' versions are really annoying.

(Kind of duplicates #41)

[stof]

Fixed by #160