Add "| null" to ElementTraversal properties

[perryjiang]

Changed

interface ElementTraversal {
    readonly childElementCount: number;
    readonly firstElementChild: Element;
    readonly lastElementChild: Element;
    readonly nextElementSibling: Element;
    readonly previousElementSibling: Element;
}

to

interface ElementTraversal {
    readonly childElementCount: number;
    readonly firstElementChild: Element | null;
    readonly lastElementChild: Element | null;
    readonly nextElementSibling: Element | null;
    readonly previousElementSibling: Element | null;
}

From issue (microsoft/TypeScript#12526).

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