Fix bug in docker-toool where values are sometimes empty.

I was getting empty volumes from the json and it was creating bad
docker command lines.

Change-Id: Ie90fc1afa5711d6b029e98d621507b9cb70c1dbe

docker/docker-toool

@@ -75,6 +75,9 @@ def parse_opts(argv):
 
 def docker_arg_map(key, value):
     value = str(value).encode('ascii', 'ignore')
+    if len(value) == 0:
+        return ''
+
     return {
         'environment': "--env=%s" % value,
         # 'image': value,