allow empty fields: "desc", "cmt", "description", "keywords", "src"

[dhontecillas]

Found an issue with a .gpx file, where the "cmt" and the "desc" fields for the waypoints were empty. It looks to me that is not a problem to have an empty comment or description. Looking at the spec, it just states that those fields must be strings (https://www.topografix.com/GPX/1/1/#type_wptType) , but there is no specification about if the have to be at least of length > 0.

My approach to fix it has been to modify the string::consume function, that accepts and additional bool parameter allow_empty. Another way would be to create a different function (consume_opt for example), that calls the original consume and checks the error. The problem with this approach is that I would need to check for the type of error returned from the string::consume function, and only if is the "no content inside string" one, return the empty string.

[frewsxcv]

thanks!