[gnome-sound-recorder] _duration undefined warning fix.

[Bilal Elmoussaoui <bilelmoussaoui src gnome org>]

commit d592f65b98be3b764a5e1914c69901ac0cb77144
Author: Kavan Mevada <kavanmevada gmail com>
Date:   Sat Apr 18 08:38:22 2020 +0000

    _duration undefined warning fix.

 src/recording.js | 5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)
---
diff --git a/src/recording.js b/src/recording.js
index 90f4115..9ff3ec8 100644
--- a/src/recording.js
+++ b/src/recording.js
@@ -60,7 +60,10 @@ var Recording = new GObject.registerClass({  // eslint-disable-line no-unused-va
     }
 
     get duration() {
-        return this._duration;
+        if (this._duration)
+            return this._duration;
+        else
+            return undefined;
     }
 
     get mimeType() {