A combined value, given a name (name) and header list (list), is the values of all headers in list whose name is a byte-case-insensitive match for name, separated from each other by 0x2C 0x20, in order.

• The call in https://fetch.spec.whatwg.org/#concept-header-list-sort-and-combine only calls it for names in the header list. That'd match an "Assert: There are some values."
• The call in https://fetch.spec.whatwg.org/#dom-headers-get explicitly returns null for a non-contained name. That'd match having "combined value" return null.
• If we interpret the specification as the Javascript [values].join(", "), we'd get "" for no-values.

I'd appreciate specifying one of these so I don't accidentally write a use that's ambiguous.