exposition format: specify precisely the semantics of empty label values

In general, a label with an empty value is identical to a label that is not set at all.
However, while that is (or should be) true in PromQL, there is a specific semantics in the exposition format (i.e. a label with an empty value does exist and only disappears after all the relabeling steps, when it is ingested into Prometheus). We should be clear about this in the spec.
