Update Migration.php

[mattschlosser]

Hi again,

I noticed an issue in v5 of the migraiton cli. When running vendor/bin/migrate down without a version number, it used to go all the way to zero, now it just shows Fatal error

Fatal error: Uncaught TypeError: ByJG\DbMigration\Migration::down(): Argument #1 ($upVersion) must be of type int, null given, called in /builds/pMYRyEFjS/0/prosper/prosperlms/vendor/byjg/migration-cli/src/DownCommand.php on line 43 and defined in /builds/pMYRyEFjS/0/prosper/prosperlms/vendor/byjg/migration/src/Migration.php:408
Stack trace:
#0 /builds/pMYRyEFjS/0/prosper/prosperlms/vendor/byjg/migration-cli/src/DownCommand.php(43): ByJG\DbMigration\Migration->down(NULL, true)
#1 /builds/pMYRyEFjS/0/prosper/prosperlms/vendor/symfony/console/Command/Command.php(279): ByJG\DbMigration\Console\DownCommand->execute(Object(Symfony\Component\Console\Input\ArgvInput), Object(Symfony\Component\Console\Output\ConsoleOutput))
#2 /builds/pMYRyEFjS/0/prosper/prosperlms/vendor/symfony/console/Application.php(1029): Symfony\Component\Console\Command\Command->run(Object(Symfony\Component\Console\Input\ArgvInput), Object(Symfony\Component\Console\Output\ConsoleOutput))
#3 /builds/pMYRyEFjS/0/prosper/prosperlms/vendor/symfony/console/Application.php(316): Symfony\Component\Console\Application->doRunCommand(Object(ByJG\DbMigration\Console\DownCommand), Object(Symfony\Component\Console\Input\ArgvInput), Object(Symfony\Component\Console\Output\ConsoleOutput))
#4 /builds/pMYRyEFjS/0/prosper/prosperlms/vendor/symfony/console/Application.php(167): Symfony\Component\Console\Application->doRun(Object(Symfony\Component\Console\Input\ArgvInput), Object(Symfony\Component\Console\Output\ConsoleOutput))
#5 /builds/pMYRyEFjS/0/prosper/prosperlms/vendor/byjg/migration-cli/scripts/migrate(23): Symfony\Component\Console\Application->run()
#6 /builds/pMYRyEFjS/0/prosper/prosperlms/vendor/bin/migrate(119): include('/builds/pMYRyEF...')
#7 {main}
  thrown in /builds/pMYRyEFjS/0/prosper/prosperlms/vendor/byjg/migration/src/Migration.php on line 408

Added null as an option for first parameter type so it doesn't crash :)

[byjg]

Thank you. PR merged, and new release created 5.0.1