Don't store empty migrations

[jsprw]

Is your feature request related to a problem? Please describe.
When running migrations automatically on file changes, it saves a migration file although nothing has changed with the following contents:

import { Migration } from 'mikro-orm';

export class Migration20200312115106 extends Migration {

  async up(): Promise<void> {

  }

}

Describe the solution you'd like
Don't store migrations without any changes, maybe have an option to disable storing empty migrations?

[B4nan]

I think we do not need an additional option, there is already a way to generate blank migration.

When you use the automatic way, it should definitely ignore the run when the schema generator returns no queries.