Skip null and undefined keys when translating via context (#6543)

* i18n: Don't translate null or undefined keys

* Add JSDoc for I18nProvider#t context fn

ui/app/helpers/higher-order-components/i18n-provider.js

@@ -15,7 +15,17 @@ class I18nProvider extends Component {
     const { localeMessages } = this.props
     const { current, en } = localeMessages
     return {
+      /**
+       * Returns a localized message for the given key
+       * @param {string} key The message key
+       * @param {string[]} args A list of message substitution replacements
+       * @return {string|undefined|null} The localized message if available
+       */
       t (key, ...args) {
+        if (key === undefined || key === null) {
+          return key
+        }
+
         return t(current, key, ...args) || t(en, key, ...args) || `[${key}]`
       },
       tOrDefault: this.tOrDefault,