Fix bugs with entrypoint/command in docker-compose run

- When no command is passed but `--entrypoint` is, set Cmd to `[]`
- When command is a single empty string, set Cmd to `[""]`

Signed-off-by: Aanand Prasad <aanand.prasad@gmail.com>

compose/cli/main.py

@@ -670,8 +670,10 @@ class TopLevelCommand(object):
                 'can not be used togather'
             )
 
-        if options['COMMAND']:
+        if options['COMMAND'] is not None:
             command = [options['COMMAND']] + options['ARGS']
+        elif options['--entrypoint'] is not None:
+            command = []
         else:
             command = service.options.get('command')