Validate imported entry to avoid error on import

We got some imports with a missing `url` field generating some errors while trying to retrieve an existing entry with that url.
Introducing the `validateEntry` allow us to dismiss a message when it doesn't have an url (or other missing stuff in the future)

src/Wallabag/ImportBundle/Import/InstapaperImport.php

@@ -105,6 +105,18 @@ class InstapaperImport extends AbstractImport
         return true;
     }
 
+    /**
+     * {@inheritdoc}
+     */
+    public function validateEntry(array $importedEntry)
+    {
+        if (empty($importedEntry['url'])) {
+            return false;
+        }
+
+        return true;
+    }
+
     /**
      * {@inheritdoc}
      */