empty auth config required for authentication-less private registries

[jgangemi]

the following is required to push images to a private registry that doesn't require authentication

<authConfig>
  <username></username>
  <password></password>
</authConfig>

which is kind of ugly and seems needless.

i thought about attempting to check if a registry has been specified and if yes but no authConfig block was found, build the empty config and use that but i'm unsure if there could be other implications from that.

i also thought about adding some kind if 'insecure' tag but that doesn't seem worth the work given it's one field replacing two and i could just as easily do this

<authConfig>
  <username>n/a</username>
  <password>n/a</password>
</authConfig>

thoughts? this may be an edge case given most registries will be secured in some manner, but maybe not.

[jgangemi]

inquired if it's safe to always send an empty encoded X-Registry-Auth in moby/moby#10983