Fix react-rte tests (#43588)
Fix react-rte tests (#43588)

They check whether a method is defined on props, but the props type
doesn't make the method optional. Typescript now catches this error, so
I made the method optional.
dsebastien/DefinitelyTyped,georgemarshall/DefinitelyTyped,dsebastien/DefinitelyTyped,mcliment/DefinitelyTyped,markogresak/DefinitelyTyped,georgemarshall/DefinitelyTyped,georgemarshall/DefinitelyTyped,georgemarshall/DefinitelyTyped