[Bug] `???` defaults cannot display `--help`

Bug

Description

--help fails when a default config option is set to ???

Checklist

• I checked on the latest version of Hydra
• I created a minimal repro (See this for tips).

To reproduce

I have something like the following for defaults:

defaults:
  - deploy: production
  - workflow: ???

Because I want to have the app run only if the "arg" configuration is given. However, when configured like this the --help command doesn't work and I get this:

hydra.errors.ConfigCompositionException: You must specify 'workflow', e.g, workflow=<OPTION>
Available options:
        dummy

I can work around this by raising my own error when this is missing, but I don't see a reason why this shouldn't work as a feature.

Expected Behavior

I would expect that at least the --help command should run rather than just dumping the error.

System information

• Hydra Version : 1.1.2
• Python version : 3.9.7
• Operating system : Ubuntu 16.04