if the result of cleaning a string is an empty string, insert a separator character

I've encountered a following problem - Pathauto removes all characters from username which contained punctuation characters only.

E.g. the user registers with username "..---" (a weird username, I know - but anyways). Pathauto is set up so that all punctuation chars are replaces with a separator ("-" in my case). I've assumed that "..---" will be converted into "-----" but it was converted to empty string leading to various issues - like blog feed alias which was "blog/feed" instead of "blog/-----/feed" for that particular user. As a result - the RSS feed for all blog entries was displaying blog entries from "..---" blog only.

I've made some debugging and found out that the problem was with the pathauto_cleanstring function - it trims leading and trailing separator characters thus producing empty string if the original string contained only punctuation characters. I've implemented a simple workaround for this - it is in the attached patch.