main.py; fix error of legacy file size that is sometimes 0

Old data ofn vidjil server have no provider field filled, and sizes where not corectly computed. In this case, value is none and can't be interpreted by metrics.

metrics/main.py

@@ -313,9 +313,11 @@ def convert_sequence_size_by_user(metrics_key, formater, data, url_server):
 # HELP vidjil_api_request_sequence_size_by_user {formater['description']}\n\
 # TYPE vidjil_api_request_sequence_size_by_user {formater['type']}\n"
     for elt in data:
+            # Some old data have none value (no provider filled); needto replace None by 0
+            size = elt['size_file_sum'] if not elt['size_file_sum'] == None else -1
             string += f"\
 vidjil_api_request_sequence_size_by_user\
-{{user_id=\"{elt['user_id']}\", server=\"{url_server}\"}} {elt['size_file_sum']}\n"
+{{user_id=\"{elt['user_id']}\", server=\"{url_server}\"}} {size}\n"
     return string