title: Rxdb migration is not running
body: <p>I have a schema with version 0, and 4k docs with it.
  Changing version to 1, have the following strategy:</p>
  <pre><code>  autoMigrate: true,
    migrationStrategies: {
      1: (r: any) =&gt; {
        debugger;
        console.log(r);
        return r;
      }
    },
  </code></pre>
  <p>reload the page, nothing is printed, debugger is not stopped.
  I tried to follow the code in <code>data-migration.js</code>, it looks like <code>getBatchOfOldCollection</code> returns an empty batch.
  What can cause it?
  I tried to run the migration manually with the same result.</p>