Fix 500 when an event thumbnail file is empty (#24110)

* fix 500 when an event thumbnail file is empty

* fix test

frigate/api/media.py

@@ -1060,7 +1060,7 @@ async def event_thumbnail(
     except DoesNotExist:
         thumbnail_bytes = None
 
-    if thumbnail_bytes is None:
+    if not thumbnail_bytes:
         # see if the object is currently being tracked
         try:
             camera_states = request.app.detected_frames_processor.get_camera_states()
@@ -1076,7 +1076,7 @@ async def event_thumbnail(
                 status_code=404,
             )
 
-    if thumbnail_bytes is None:
+    if not thumbnail_bytes:
         return JSONResponse(
             content={"success": False, "message": "Event not found"},
             status_code=404,
@@ -1085,6 +1085,13 @@ async def event_thumbnail(
     img_as_np = np.frombuffer(thumbnail_bytes, dtype=np.uint8)
     img = cv2.imdecode(img_as_np, flags=1)
 
+    if img is None:
+        # thumbnail on disk is truncated or corrupt
+        return JSONResponse(
+            content={"success": False, "message": "Event not found"},
+            status_code=404,
+        )
+
     # android notifications prefer a 2:1 ratio
     if format == "android":
         img = cv2.copyMakeBorder(