no-vary: simplify SF field format

[reschke]

Section 3:

If present, the params entry's value MUST be either a boolean (Section
3.3.6 of [STRUCTURED-FIELDS]) or an inner list (Section 3.1.1 of
[STRUCTURED-FIELDS]).

Maybe personal taste, but having varying datatypes for the same param feels wrong. Maybe use empty list instead
of boolean.

That would change:

No-Vary-Search: params, except=("productId")

to

No-Vary-Search: params=(), except=("productId")

which would make things both more consistent and readable. Or even drop
the "no params" syntax.

No-Vary-Search: except=("productId")

Also: this should say what format the contents of the inner list can have. (just stating it in the parsing algorithm is not suffient)

(see https://lists.w3.org/Archives/Public/ietf-http-wg/2025OctDec/0000.html)

[nidhijaju]

My understanding is that params=?1 means no-vary params are set to wildcard, so I don't like having an empty list represent wildcard, when no-vary params can also technically be an empty list.

Took a stab at changing to use the "no params" syntax in #3374, but I'm not sure if it's actually better. Would love to hear everyone's thoughts.