fix: set episode guid upon episode creation

fixes #48

app/Controllers/Admin/Episode.php

@@ -111,6 +111,7 @@ class Episode extends BaseController
             'podcast_id' => $this->podcast->id,
             'title' => $this->request->getPost('title'),
             'slug' => $this->request->getPost('slug'),
+            'guid' => '',
             'enclosure' => $this->request->getFile('enclosure'),
             'description' => $this->request->getPost('description'),
             'image' => $this->request->getFile('image'),

app/Entities/Episode.php

@@ -199,7 +199,7 @@ class Episode extends Entity
         );
     }
 
-    public function setGuid($guid = null)
+    public function setGuid(string $guid)
     {
         return $this->attributes['guid'] = empty($guid)
             ? $this->getLink()