fix serialize restart spec with null string

Signed-off-by: realityone <realityone@me.com>

compose/config/types.py

@@ -93,6 +93,8 @@ def parse_restart_spec(restart_config):
 
 
 def serialize_restart_spec(restart_spec):
+    if not restart_spec:
+        return ''
     parts = [restart_spec['Name']]
     if restart_spec['MaximumRetryCount']:
         parts.append(six.text_type(restart_spec['MaximumRetryCount']))

tests/acceptance/cli_test.py

@@ -236,6 +236,10 @@ class CLITestCase(DockerClientTestCase):
                     'image': 'busybox',
                     'restart': 'on-failure:5',
                 },
+                'restart-null': {
+                    'image': 'busybox',
+                    'restart': ''
+                },
             },
             'networks': {},
             'volumes': {},

tests/fixtures/restart/docker-compose.yml

@@ -12,3 +12,6 @@ services:
   on-failure-5:
     image: busybox
     restart: "on-failure:5"
+  restart-null:
+    image: busybox
+    restart: ""