fix(i18n): prevent infinite loop when fetching bundles (#1333)

Prevent the possibility of infinite loop, if localeId is null, undefined, etc.:

FIXES: #1318

Author: ilhan007

packages/base/src/locale/nextFallbackLocale.js

@@ -2,11 +2,11 @@
  * Calculates the next fallback locale for the given locale.
  *
  * @param {string} locale Locale string in Java format (underscores) or null
- * @returns {string|null} Next fallback Locale or null if there is no more fallback
+ * @returns {string} Next fallback Locale or "en" if no fallbacks found.
  */
 const nextFallbackLocale = locale => {
 	if (!locale) {
-		return null;
+		return "en";
 	}
 
 	if (locale === "zh_HK") {

packages/base/src/locale/normalizeLocale.js

@@ -12,11 +12,15 @@ const M_ISO639_NEW_TO_OLD = {
 /**
  * Normalizes the given locale in BCP-47 syntax.
  * @param {string} locale locale to normalize
- * @returns {string} Normalized locale or undefined if the locale can't be normalized
+ * @returns {string} Normalized locale, "undefined" if the locale can't be normalized or "en" if no locale provided.
  */
 const normalizeLocale = locale => {
 	let m;
 
+	if (!locale) {
+		return "en";
+	}
+
 	if (typeof locale === "string" && (m = localeRegEX.exec(locale.replace(/_/g, "-")))) {/* eslint-disable-line */
 		let language = m[1].toLowerCase();
 		let region = m[3] ? m[3].toUpperCase() : undefined;