Crashes on AJV initialized with `meta: false`

Following will crash with not very descriptive error `Error: can't resolve reference http://json-schema.org/draft-07/schema# from id #`

```javascript
const AJV = require('ajv');
const ajv = new AJV({ meta: false });
require('ajv-merge-patch')(ajv);
```

If it works as intended it'll be good to document that AJV cannot be initialized with `meta: false`, if we want to take advantage of this plugin