wurzelitzer: Check whether SD card is present

python_payload/apps/wurzelitzer/__init__.py

-import st3m.run, st3m.wifi, media, os, ctx
+import st3m.run, st3m.wifi, media, uos, ctx
 from st3m.application import Application
+from st3m.utils import sd_card_plugged
 
 RADIOSTATIONS = [
     "http://radio-paralax.de:8000/",
@@ -16,7 +17,8 @@ class App(Application):
     def __init__(self, app_ctx):
         super().__init__(app_ctx)
         self._streams = RADIOSTATIONS.copy()
-        for entry in os.ilistdir("/sd/"):
+        if sd_card_plugged():
+            for entry in uos.ilistdir("/sd/"):
                 if entry[1] == 0x8000:
                     if (
                         entry[0].endswith(".mp3")