Fix preview retrieval to handle missing previews gracefully (#22331)

frigate/api/media.py

@@ -1531,6 +1531,7 @@ def preview_gif(
 ):
     if datetime.fromtimestamp(start_ts) < datetime.now().replace(minute=0, second=0):
         # has preview mp4
+        try:
             preview: Previews = (
                 Previews.select(
                     Previews.camera,
@@ -1548,8 +1549,7 @@ def preview_gif(
                 .limit(1)
                 .get()
             )
-
-        if not preview:
+        except DoesNotExist:
             return JSONResponse(
                 content={"success": False, "message": "Preview not found"},
                 status_code=404,