marshal: correctly handle empty shell commands

It's possible to override an image's entrypoint for service
container's by setting `service.entrypoint`. (Same applies for
image's command.)

In some circumstances, it's desirable to explicitly "clear"
these out from the image. However, currently, if you do specify
`entrypoint: ''` or `entrypoint: []`, when the config is
marshaled, that gets lost because of Go default value handling.

Handling this varies slightly between YAML + JSON:
 * YAML: implement `IsZeroer` interface to specify that only
   `nil` slices should be omittable & `MarshalYAML` to ensure
   that `nil` slices marshal as `null` rather than `[]`
 * JSON: do NOT use `omitempty`, meaning that the result will
   _always_ be present in the marshaled output, unfortunately,
   but it will be correct!

This preserves compatibility and "pretty" serialization for
YAML at the expense of some slightly subtle marshalling
semantics.

Signed-off-by: Milas Bowman <milas.bowman@docker.com>