Broken in 11c84973ef. The section to skip
an empty input was accidentally removed when some code was refactored to
fix a separate issue.
This skips empty cache entries which allows disabling the `cache-from` and
`cache-to` entries from the command line overrides.
Signed-off-by: Jonathan A. Sternberg <jonathan.sternberg@docker.com>
6019a2b32f