support empty arch field -> skip build

@PureTryOut proposed in https://gitlab.com/postmarketOS/pmaports/-/commit/2919f372ac493689ead8759cc27f9a9ec9a41c83#note_389794406, that we use arch="" (empty arch field) in APKBUILD to indicate that a package should not be built - without deleting it completely. The advantage appears to be a better git history.

Currently, pmbootstrap is complaining about this, e.g. here:

$ pmbootstrap repo_missing --built hello-world
[15:03:08] Calculate packages that need to be built (hello-world, x86_64)
[15:03:08] ERROR: Arch must not be empty: /home/user/.local/var/pmbootstrap/cache_git/pmaports/main/hello-world/APKBUILD
[15:03:08] See also: <https://postmarketos.org/troubleshooting>
Run 'pmbootstrap log' for details.

AFAIK it should work with bpo, if we remove the check from pmbootstrap here. But before closing this issue, we should make sure it actually works with bpo.