This matches the prior behavior before 2a6ff3c24f.
This also updates the Swagger documentation for the current version to note that the field might be the empty string and what that means.
Signed-off-by: Tianon Gravi <admwiggin@gmail.com>
(cherry picked from commit b4fbe226e8)
Signed-off-by: Bjorn Neergaard <bjorn.neergaard@docker.com>