Don't send empty string for required param

[bbangert]

Per Sentry issue, https://sentry.prod.mozaws.net/operations/autopush-prod/issues/383827/

ValidationException: ValidationException: 400 Bad Request
{u'__type': u'com.amazon.coral.validate#ValidationException', u'message': u'ExpressionAttributeValues contains invalid value: One or more parameter values were invalid: An AttributeValue may not contain an empty string for key :router_data'}

Traceback:

ValidationException: ValidationException: 400 Bad Request
{u'__type': u'com.amazon.coral.validate#ValidationException', u'message': u'ExpressionAttributeValues contains invalid value: One or more parameter values were invalid: An AttributeValue may not contain an empty string for key :router_data'}
  File "twisted/python/threadpool.py", line 246, in inContext
    result = inContext.theWork()
  File "twisted/python/threadpool.py", line 262, in <lambda>
    inContext.theWork = lambda: context.call(ctx, func, *args, **kw)
  File "twisted/python/context.py", line 118, in callWithContext
    return self.currentContext().callWithContext(ctx, func, *args, **kw)
  File "twisted/python/context.py", line 81, in callWithContext
    return func(*args,**kw)
  File "autopush/db.py", line 292, in wrapper
    return func(self, *args, **kwargs)
  File "autopush/db.py", line 718, in register_user
    return_values="ALL_OLD",
  File "boto/dynamodb2/layer1.py", line 2765, in update_item
    body=json.dumps(params))
  File "boto/dynamodb2/layer1.py", line 2842, in make_request
    retry_handler=self._retry_handler)
  File "boto/connection.py", line 954, in _mexe
    status = retry_handler(response, i, next_sleep)
  File "boto/dynamodb2/layer1.py", line 2882, in _retry_handler
    response.status, response.reason, data)

[pjenvey]

already fixed in #870, test added in #881