[jsoneditor] Enable strictNullChecks (#43828)
[jsoneditor] Enable strictNullChecks (#43828)


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