Fix MIDIMessageEvent.data incorrectly typed as nullable

[Copilot]

MIDIMessageEvent.data was typed as Uint8Array | null, forcing unnecessary null checks (TS18047) under --strict. Per the Web MIDI spec, data is always a non-null Uint8Array.

Changes

• Updated MIDIMessageEvent.data in tsc/internal/bundled/libs/lib.dom.d.ts to Uint8Array<ArrayBuffer> (removed the | null union)
• Applied the same fix to the mirrored test fixture tsc/testdata/fixtures/lib/dom.generated.d.ts
• Added compiler test midiMessageEventDataNotNullable.ts verifying event.data[0] no longer errors under --strict in a midimessage handler

input.onmidimessage = (event) => {
    event.data[0]; // no longer errors: Object is possibly 'null'
};

• Fixes MIDIMessageEvent data is incorrectly typed #60546