Define asset url if it does not exist

[caleb-mabry]

When the yaml is parsed from the config, if the variable is not defined it will come out as an empty string. I want to keep the behavior of the empty string rather than setting it as None and having to check it in other place.

EDIT: I was wrong, if the value is empty in the yaml pyyaml sets it to None. This change was reflected in the latest commit

[stonedDiscord]

this still sends None if asset_url is present in config.yaml but has nothing set after it like in the sample config

[caleb-mabry]

@stonedDiscord Great catch! I've fixed it in my latest commit.