Added null check for Issue # 2681

Author: prabhupad26

mongoengine/base/fields.py

@@ -511,7 +511,8 @@ def to_python(self, value):
     def to_mongo(self, value):
         if not isinstance(value, ObjectId):
             try:
-                return ObjectId(str(value))
+                if value:
+                    return ObjectId(str(value))
             except Exception as e:
                 self.error(str(e))
         return value