add possibility to omit config

[alex-popov-tech]

Hey guys, thank you for the plugin!
I was struggling for 2 days trying to get it working with configuring typeorm through env variables, and discovered that fastify always passing {} to async function typeormConnector (fastify, options) { for options, which makes it call createConnection with that empty object even if user registered plugin without passing it. And typeorm.createConnection({}) seems to trying to find config in object passed and stucks, so basically that plugin does not support env-like setup. I've added couple of guards to check if passed object is empty, treat it as undefined passed to createConnection, added tests for it, updated documentation, and tested locally with my fastify project as well.
Looking forward to merge it, since its blocking my app from using env variables for typeorm with this cool plugin. :)