Allow creation of empty files

- change to a pointer type so we can distinguish empty from unset.

Signed-off-by: Justin Cormack <justin.cormack@docker.com>

test/test.yml

@@ -29,6 +29,8 @@ services:
 files:
   - path: etc/docker/daemon.json
     contents: '{"debug": true}'
+  - path: /empty
+    contents: ""
 trust:
   org:
     - library