issue ngx-translate#971 handling empty default language string

An empty string is now also allowed as default language.
It checks if the default language is null. 
The check for 'undefined' is implied because of the use of == instead of ===.

projects/ngx-translate/core/src/lib/translate.service.ts

@@ -167,7 +167,7 @@ export class TranslateService {
 
     if (typeof pending !== "undefined") {
       // on init set the defaultLang immediately
-      if (!this.defaultLang) {
+      if (this.defaultLang == null) {
         this.defaultLang = lang;
       }
 
@@ -340,7 +340,7 @@ export class TranslateService {
       res = this.parser.interpolate(this.parser.getValue(translations, key), interpolateParams);
     }
 
-    if (typeof res === "undefined" && this.defaultLang && this.defaultLang !== this.currentLang && this.useDefaultLang) {
+    if (typeof res === "undefined" && this.defaultLang != null && this.defaultLang !== this.currentLang && this.useDefaultLang) {
       res = this.parser.interpolate(this.parser.getValue(this.translations[this.defaultLang], key), interpolateParams);
     }
 
@@ -457,7 +457,7 @@ export class TranslateService {
     this.onLangChange.emit({lang: lang, translations: this.translations[lang]});
 
     // if there is no default lang, use the one that we just set
-    if (!this.defaultLang) {
+    if (this.defaultLang == null) {
       this.changeDefaultLang(lang);
     }
   }