Skip domain `_` (Nginx default domain name)

Hi,

Latest version of Dokku 0.30.4 introduced a new feature dokku/dokku#5851 where it's possible to set an underscore as a domain to allow any domain to reach an app.

I believe dokku-letsencrypt should skip this domain instead of raise an error such as

-----> Enabling ACME proxy for app_name...
-----> Getting letsencrypt certificate for app_name via HTTP-01
        - Domain 'admin.appname.com'
        - Domain 'appname.me'
        - Domain '_'
2023/05/16 13:02:43 No key found for account email@example.com. Generating a P256 key.
2023/05/16 13:02:43 Saved key to /certs/accounts/acme-v02.api.letsencrypt.org/email@example.com/keys/email@example.com.key
2023/05/16 13:02:43 [INFO] acme: Registering account for email@example.com
2023/05/16 13:02:44 [INFO] [admin.appname.com, appname.me, _] acme: Obtaining bundled SAN certificate
       !!!! HEADS UP !!!!

       Your account credentials have been saved in your Let's Encrypt
       configuration directory at "/certs/accounts".

       You should make a secure backup of this folder now. This
       configuration directory will also contain certificates and
       private keys obtained from Let's Encrypt so making regular
       backups of this folder is ideal.
2023/05/16 13:02:44 Could not obtain certificates:
	acme: error: 400 :: POST :: https://acme-v02.api.letsencrypt.org/acme/new-order :: urn:ietf:params:acme:error:rejectedIdentifier :: Error creating new order :: Cannot issue for "_": Domain name contains an invalid character
-----> Certificate retrieval failed!
-----> Disabling ACME proxy for app_name...
 !     Failed to setup letsencrypt
 !     Check log output for further information on failure