mod validation is overly strict

[duk3luk3]

When uploading a mod via the python-api with url = "" in the mod_info.lua, this exception is thrown:

  File "/code/api/mods.py", line 85, in mods_upload
    process_uploaded_mod(temp_mod_path)
  File "/code/api/mods.py", line 206, in process_uploaded_mod
    mod_info = parse_mod_info(temp_mod_path)
  File "/usr/local/lib/python3.5/site-packages/faf/tools/fa/mods.py", line 52, in parse_mod_info
    raise ValueError(errors)
ValueError: {'url': ['Not a valid URL.']}

That is probably too strict.