[xregexp] Enable strictNullChecks (#43639)
[xregexp] Enable strictNullChecks (#43639)


dsebastien/DefinitelyTyped,dsebastien/DefinitelyTyped,georgemarshall/DefinitelyTyped,georgemarshall/DefinitelyTyped,georgemarshall/DefinitelyTyped,mcliment/DefinitelyTyped,georgemarshall/DefinitelyTyped,markogresak/DefinitelyTyped