url: fix URLSearchParams(null) per spec

[marcopiraccini]

url: fix URLSearchParams(null) to produce "null=" per spec

Per the WHATWG URL spec, the URLSearchParams constructor accepts a (sequence<sequence<USVString>> or record<USVString, USVString> or USVString) union. null is not a sequence or record, so WebIDL union resolution falls through to USVString, where ToString(null) is "null".

Here we short-circuited on init == null, treating null the same as undefined and producing an empty string.

Before:

new URLSearchParams(null).toString() // ''

After (matches Chrome and Deno):

new URLSearchParams(null).toString() // 'null='

Fixes: #63559

[nodejs-github-bot]

Review requested:

• @nodejs/url

[mcollina]

lgtm

[nodejs-github-bot]

CI: https://ci.nodejs.org/job/node-test-pull-request/73935/