fix: don't throw when a set-cookie string has no name-value-pair

[spokodev]

parseString (and therefore parseSetCookie) throws when a Set-Cookie string contains only semicolons/whitespace and no name-value-pair:

parseSetCookie(";")
// TypeError: Cannot read properties of undefined (reading 'split')

parseSetCookie([";", "foo=bar"])
// throws — and the valid foo=bar cookie is lost along with it

setCookieValue.split(";").filter(isNonEmptyString) yields an empty array for such input, so parts.shift() is undefined and parseNameValuePair(undefined) calls .split on it.

This is inconsistent with how the library already handles empty content: parseSetCookie("") and parseSetCookie(" ") return [], and invalid cookies (e.g. forbidden __proto__ names) are dropped rather than throwing. A string that is all semicolons has no name-value-pair, so per RFC 6265 §5.2 it should be ignored, not crash the parser.

Fix: return null when there is no name-value-pair, so it's filtered out exactly like the other ignored-cookie cases. parseSetCookie(";") now returns [], parseSetCookie([";", "foo=bar"]) returns just the valid cookie, and { map: true } returns {}.

Adds a regression test.

[nfriedly]

Thank you, this is great!

[nfriedly]

This is now published to npm in v3.1.2