fix: [#2356] Ignore null and undefined when setting input.files

[mixelburg]

Setting input.files = null (or undefined) on a file input is stored as-is, so reading .value afterwards throws. Per spec these assignments are no-ops, and browsers keep the existing FileList — this also matches jsdom, which ignores the write when the input isn't a file type.

The setter now returns early for null/undefined and for non-file inputs. Added tests covering both cases.

Closes #2356

[mixelburg]

Hi! The build (20) failure here looks unrelated to this change — it's in @happy-dom/server-renderer (ServerRendererBrowser.test.ts / ServerRendererPage.test.ts), specifically the timing-sensitive waitUntilComplete() tests: "2 tasks did not end in time" vs the expected "1 task", plus a null .error in the debug-error case. This PR only touches the HTMLInputElement.files setter and its tests.

The sibling PR #2362 ran the same server-renderer tests on Node 20 the same day and passed, which points to a flake rather than a real regression. Would you mind re-running the build (20) job? Happy to dig in further if it turns out not to be a flake.