fix: use == null instead of === undefined for window.event check

[linsk1998]

Summary

The resolveUpdatePriority function in ReactDOMUpdatePriority.js uses currentEvent === undefined to check if window.event is absent. However, window.event returns null (not undefined) in Internet Explorer 10, causing the strict equality check to fail. When this happens, getEventPriority(null.type) is called, which throws TypeError: Cannot read property 'type' of null.

Changes

• Changed currentEvent === undefined to currentEvent == null to cover both null (IE10) and undefined (IE11)
• Added a comment documenting the browser behavior difference

Browser Compatibility

Browser	window.event value
Modern browsers	undefined (when no event is being handled)
IE11	undefined
IE10	null

Using == null (loose equality) correctly handles both cases, as it matches both null and undefined.

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