builder: fix incorrect cache match for inline cache with empty layers
See https://github.com/moby/buildkit/pull/1993
Signed-off-by: Tibor Vass <tibor@docker.com>
(cherry picked from commit 9bf93e90fa9ce74b6f7309a22838da8189838b3b)
Signed-off-by: Tibor Vass <tibor@docker.com>