add --if-exists flag to migrate command

[karadnik-flutterint]

Fixes #127

this is to prevent the action from exiting with error when there are no migrations generated from the current migration, which is a perfectly valid case

[karadnik-flutterint]

Hm, build-test action fails. It looks like an access issue?

remote: Permission to timonmasberg/nx-migration-gh-action.git denied to github-actions[bot].
fatal: unable to access 'https://github.com/timonmasberg/nx-migration-gh-action/': The requested URL returned error: 403
Error: The process '/usr/bin/git' failed with exit code 128

[timonmasberg]

Thank you! Didnt know about this case.