rx matching empty string

[mmoskal]

We currently fail here, for simple rx like '\d*'

impl FunctionalRecognizer<RecRxState> for RecRx {
    fn initial(&self) -> RecRxState {
        self.dfa
            .universal_start_state(regex_automata::Anchored::Yes)
            .expect("dfa has no universal start state; make sure it doesn't match empty string")
    }