signature attribute; small cleanup

[traversc]

Two small changes to pr/1183 based on @kevinushey 's comments.

1. Check sig.empty() before calling sig.front() and sig.back()

2. Change -1 to std::string:npos for stylistic reasons.

Checks using github actions still pass: https://github.com/traversc/Rcpp/runs/4003616100?check_suite_focus=true

[eddelbuettel]

Looks good. Let me wrap up what we have now in #1183 and throw it again at the rev.dep machine. Not expecting anything nefarious as unit tests rarely call compileAttributes() but it does not hurt to stick with the process.