moment.locale() returns undefined after updating a non-existing locale

[keqingrong]

Describe the bug
moment.locale() returns undefined after updating a non-existing locale.

To Reproduce

moment.locale('en');
console.log(moment.locale()); // 'en'
moment.updateLocale('dude', {});
console.log(moment.locale()); // undefined

Expected behavior
moment.updateLocale() should ignore the non-existing locale or fallback to moment.defineLocale(), and gives a warning just like what moment.defineLocale() did .

moment.locale('en');
console.log(moment.locale()); // 'en'
moment.updateLocale('dude', {});
// Deprecation warning: use moment.defineLocale(localeName, config) to create
// a new locale. moment.updateLocale(localeName, config) should only be used
// for changing an existing locale.
console.log(moment.locale()); // 'en' or 'dude'

Moment version
2.24.0