Prohibit an empty string Dictionary-ID

[horo-t]

Dictionary-ID was introduced to the Compression Dictionary Transport specification by #2687.

However, it is unclear whether empty string dictionary IDs are allowed.
I don't think there is any use case for an empty string dictionary ID.
So how about explicitly disallowing empty string dictionary IDs in the spec?
@pmeenan

[pmeenan]

I'll make a zero-length string the default and have that mean "no ID". The dictionary ID will only be sent if it has a length > 0.