[SkygearCloud] Crash if DATABASE_URL doesn't have auth

[limouren]

If supplied with DATABASE_URL without auth (e.g. postgresql:///mydb, which is a valid connection string accepted by psql), skygear-node crashes.

> DATABASE_URL=postgresql:///mydb ./node_modules/.bin/skygear-node
/Users/limouren/myproject/node_modules/skygear-core/dist/cloud/pg.js:38
  var auth = params.auth.split(':');
                         ^

TypeError: Cannot read property 'split' of null
    at databaseConfigFromURL (/Users/limouren/myproject/node_modules/skygear-core/dist/cloud/pg.js:38:26)
    at Object.<anonymous> (/Users/limouren/myproject/node_modules/skygear-core/dist/cloud/pg.js:51:14)
    at Module._compile (internal/modules/cjs/loader.js:738:30)
    at Object.Module._extensions..js (internal/modules/cjs/loader.js:749:10)
    at Module.load (internal/modules/cjs/loader.js:630:32)
    at tryModuleLoad (internal/modules/cjs/loader.js:570:12)
    at Function.Module._load (internal/modules/cjs/loader.js:562:3)
    at Module.require (internal/modules/cjs/loader.js:667:17)
    at require (internal/modules/cjs/helpers.js:20:18)
    at Object.<anonymous> (/Users/limouren/myproject/node_modules/skygear-core/dist/cloud/transport/common.js:38:11)

• Skygear SDK Date/Version: 1.7.1
• JS runtime: v11.10.0
• Skygear Server Date/Version: un-related
• Is this a regression?
• Attached logs, screenshots

Expected Results

It starts normally.

Actual Results

It crashes.

Steps to reproduce

Given already in the description.