run-buildset-container: fix username/password for proxy registry

This configuration is intended to mask the creds supplied by the
local docker configuration when the registry is used as a
pull-through proxy to upstream.  But we need to set the creds
to the empty string, rather than null, which ends up as the string
"None" in the docker config.

Change-Id: Ic36a5c764dec8df4fa460f6735ff5668a1dd3b50

roles/run-buildset-registry/tasks/main.yaml

@@ -74,8 +74,8 @@
       REGISTRY_AUTH_HTPASSWD_PATH: /auth/htpasswd
       REGISTRY_AUTH_HTPASSWD_REALM: Registry Realm
       REGISTRY_PROXY_REMOTEURL: https://registry-1.docker.io
-      REGISTRY_PROXY_USERNAME:
-      REGISTRY_PROXY_PASSWORD:
+      REGISTRY_PROXY_USERNAME: ''
+      REGISTRY_PROXY_PASSWORD: ''
     volumes:
       - "{{ buildset_registry_root}}/data:/var/lib/registry"
       - "{{ buildset_registry_root}}/certs:/certs"