Use the same default timestamp for converted data

Since the conversion is logically one moment in time then the timestamp
used for records that do not have one should be the same.

src/mdyml/convert_cisagov.py

@@ -87,6 +87,11 @@ def convert() -> None:
             table_rows.append(line)
     logging.info("Done reading Markdown table with %d rows.", len(table_rows))
 
+    # The timestamp to use if there is no timestamp in the row. Since the
+    # conversion is logically one action it should be the same for all converted
+    # data.
+    default_timestamp = datetime.now(timezone.utc).isoformat(timespec="seconds")
+
     # Process all the data into a list of dictionaries
     out_dict_list = []
     row_count = 0
@@ -155,9 +160,7 @@ def convert() -> None:
                 parsed_date.replace(tzinfo=timezone.utc)
             out_dict["last_updated"] = parsed_date.isoformat(timespec="seconds")
         else:
-            out_dict["last_updated"] = datetime.now(timezone.utc).isoformat(
-                timespec="seconds"
-            )
+            out_dict["last_updated"] = default_timestamp
 
         out_dict_list.append(out_dict)