Improve approach to valid cases where some data is empty from charmhub

[Heather Lanigan]

Rather than guard against potential bad data when we unmarshal charmhub responses, solve the problem where the data is received.

Example core/charm/repository/charmhub.go:

 // NOTE: Charmhub returns a "{}\n" when no config.yaml exists for
 // the charm, e.g. postgreql. However, this will fail the charm
 // config validation which happens in ReadConfig. Valid config
 // are nil and "Options: {}"
 if configYAML != "" || strings.TrimSpace(configYAML) == "{}" {
  chConfig, err = charm.ReadConfig(strings.NewReader(configYAML))
  if err != nil {
   return corecharm.EssentialMetadata{}, errors.Annotatef(err, "parsing config.yaml for %q", curl)
  }
 }

[Ian Booth (wallyworld)]

The next 2.9.46 candidate release will not include a fix for this bug and we don't plan on any more 2.9 releases. As such it is being removed from its 2.9 milestone.

If the bug is still important to you, let us know and we can consider it for inclusion on a 3.x milestone.