In license preference, replace blank entry with "default"

[cellio]

https://meta.codidact.com/posts/287774

In user preferences there is a community-specific preference for the default license. It looks like this:

The top option is a blank line, which means to use the default. It doesn't mean no default; that's the last entry. Let's make this clearer by replacing that blank line with "category/site default".

The linked post describes a problem with the blank line being very short and thus easy to miss. I can't reproduce that, but adding text here would presumably also fix that if it's still happening.

[cellio]

This setting comes from config/config/preferences.yml, where I see default: ~, and grep is not the right tool for finding the other side of this.