event-target: make AddOptions type compatible with recent TS

[peat-psuwit]

Recent update in TS add signal property to AddOptions interface [1]. Over there, the definition is:

    signal?: AbortSignal;

You'll notice that null is not in the definition. This causes the whole EventTarget type to be incompatible with TS one.

To fix this incompatibility, just remove null from the field's union.

[1] https://github.com/microsoft/TypeScript/blob/feac9eb126e56837d16acb61cd019ce8520db76c/src/lib/dom.generated.d.ts#L8