"I don't want this to match anything" frontmatter

Right now in the frontmatter (but not in the CLI args), if you specify an empty selector, it treats this as unset. We should instead keep this consistent and make an empty array indicative of "I don't want this to match anything".

For example:

task_name: example
selectors:
  foo: []
Right now would match foo:bar and foo:baz in rules. It should match nothing.

