feat(middleware): add helpers for authorization middleware

Closes #1 (closed)

src/graphqlSchema.js

 const config = require('config')
-
+const isEmpty = require('lodash/isEmpty')
 const { applyMiddleware } = require('graphql-middleware')
 const { shield } = require('graphql-shield')
 let schema = require('pubsweet-server/src/graphql/schema')
 
-if (config.has('permissions')) {
-  schema = applyMiddleware(schema, shield(config.get('permissions')))
+const permissions = config.has('permissions') && config.get('permissions')
+
+if (permissions && !isEmpty(permissions)) {
+  schema = applyMiddleware(schema, shield(permissions))
 }
 
 module.exports = schema