Fix import opml without folders.

When OPML doesn't contain folders (all feeds are located in root) app can't import it. I've fixed it.

For reproducing try to import this OPML:

<?xml version="1.0" encoding="UTF-8"?>
<opml version="2.0">
<head>
<title>Feed Subscriptions</title>
</head>
<body>
      <outline text="The Kotlin Blog" title="The Kotlin Blog" type="rss"
            xmlUrl="https://blog.jetbrains.com/kotlin/feed/" htmlUrl="https://blog.jetbrains.com"/>
</body>
</opml>

readrops / Readrops

Fix import opml without folders. #149