Add test vector for [null, true, ...]

[cabo]

[Section 5.1]

• "two leading null values (scheme and authority both not given) MUST be represented by using the discard alternative instead, and"

  Is this actually the only case where to use discard?

  Scheme and authority are both not given also in case the first two elements in the outer array are null (absent scheme) and true (absent authority followed by rootless path). Shouldn't discard be used also in this case?

[_]
In some cases, you would use [1, [some-path]] instead of [null, true, [some-path]], but not when you want to explicitly make the CRI rootless.
(We don't have an example where this would make sense, but it currently is possible.)
In any case, using discard would make it impossible to select NOAUTH-ROOTLESS.
➔ add a test vector