Add non-emptiness to conclusion of positive RE star unfolding. (#4899)

Add non-emptiness to conclusion of positive RE star unfolding. (#4899)

A recent commit (77e9881) simplified the form of the conclusion in regular expression star unfolding for the sake of uniformity in our internal proof calculus. However, this led to a noticeable drop in performance on a few specific RE benchmarks (the Norn set). This preserves the old behavior by extending the core rule for RE unfolding.

It also makes one minor change to the strings proof checker carried over from the proof-new branch.