Kong should not emit an empty string as value of `http.route`

[mmanciop]

In the instrumentation, the empty string is the fallback for a missing value for the http.route attribute to, probably, signify "not known". That is not a correct use of the http.route attribute: if the route is not known, the attribute must be not set on the span.

[tzssangglass]

this comes from: #8125