test: add util.inspect test for null maxStringLength

Add test case to cover currently-uncovered code.

Refs: https://coverage.nodejs.org/coverage-39a7f7663e8f70fc/lib/internal/util/inspect.js.html#L333

PR-URL: https://github.com/nodejs/node/pull/36086


Reviewed-By: Antoine du Hamel <duhamelantoine1995@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Daijiro Wachi <daijiro.wachi@gmail.com>