Regression: null translation values no longer fallback to default language in v17

[jvuor]

After upgrading to v17 of ngx-translate, I noticed that translation keys with null values are no longer replaced by the fallback language translation. Instead, they trigger the MissingTranslationsHandler and display the translation key. This differs from previous versions, where the fallback language was used if a translation was null.

Current behavior

In v17, when a translation value is null, it is not replaced by the default language translation (as it was prior to v17). Instead, it triggers the MissingTranslationsHandler.

Expected behavior

When a translation value is null, I expect fallbackLang to be used and the translation to be taken from the fallback language file.

Minimal reproduction of the problem with instructions

• Set fallbackLang in v17.
• Load a set of translations where some translation values are null.
• Observe that these keys are not replaced by the fallback language translation; instead, the translation key is displayed.