Default language with empty string is overwritten to current language when it should be used

[JuleSch]

Current behavior

The string of my default language, is an empty string.
If I use a language that contains a missing translation, the default language is normally used.
But if the default language consists an empty string (in my case), it will be overwritten with the used language.

Expected behavior

The default language should not be overwritten.

How do you think that we should fix this?

In the file ngx-translate/core/src/lib/translate.service.ts there are three queries for existing default language.
Lines 170 and 460 contain
if (!this.defaultLang)
and in line 343
if (typeof res === "undefined" && this.defaultLang ...

These queries should be changed to:
if (this.defaultLang == null)
and to
if (typeof res === "undefined" && this.defaultLang != null...

Minimal reproduction of the problem with instructions

app.component.ts

constructor(private translate: TranslateService) {
translate.setDefaultLang("");
translate.use("");
translate.addLangs(['de', 'en']);
}

changeLang(lang){
this.translate.use(lang);
}

If the method changeLang() is called with the language 'de' and the translation key in 'de' does not exist, the system does not switch to the default language, but simply displays the key string.

Environment

ngx-translate version: 10.0.2
Angular version: 6.1.8


Browser:
- [x ] Chrome (desktop) version 62.0.3202.89
 
For Tooling issues:
- Node version: 10.7.0
- Platform:  ubuntu 16.04 LTS