Commit 2b17f4c8a8 fixed the way empty labels
are taken into account (i.e. not interpolated from environment variable),
but it created a regression.
`ValidateLabel` functions doesn't allow empty label value, but it has
always been possible to pass an empty label via the cli (`docker run --label foo`).
This fixes that by not validating the label flag.
Signed-off-by: Vincent Demeester <vincent@sbr.pm>
31dc5c0a9a