Fix locked

Author: dshabin

src/superannotate/lib/core/helpers.py

@@ -139,7 +139,7 @@ def safe_time(timestamp):
         return "0" if str(timestamp) == "0.0" else timestamp
 
     def convert_timestamp(timestamp):
-        return timestamp / 10 ** 6 if timestamp else None
+        return timestamp / 10 ** 6 if timestamp else "0"
 
     editor_data = {
         "instances": [],