Fix for #4693 -- implicit stages should never be serialized

My fix for #4542 was overly enthusiastic about assuring that all resources had
a stage, resulting in stages designations being serialized for resources in
manifests which did not use resources (everything was in implicit main). This
broke 0.25.x compatibility, as all catalogs now refered to stages.

This patch scales back the change for #4542 slightly, supressing the setting of
main on the puppetmaster and relying on the default behaviour on the client (for
2.6.x and later, treat it as main; for 0.25.x, do nothing).