Semantics of absent header field versus header field with empty value

In light of the discussion regarding support for absent header fields in HTTP Message Signatures<https://github.com/httpwg/http-extensions/pull/1976>, I'd like to ask the perhaps obvious question: does HTTP permit header fields to make a semantic distinction between the header field being absent from a message, and the header field being present with field value equal to the empty string? If so, are there real-world use cases that people are aware of for this that might help inform that discussion?

Thanks!

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Annabelle Backman (she/her)
richanna@amazon.com<mailto:richanna@amazon.com>