Add string polyfills

Author: dweber019

src/app/main.ts

@@ -28,6 +28,11 @@ import de_CHTranslation from './../locales/de_CH.json';
 import 'isomorphic-fetch';
 import LanguageDetector from 'i18next-browser-languagedetector';
 
+/**
+ * Polyfills
+ */
+import 'utils/polyfills.utils';
+
 // Fontawesome setup
 import fontawesome from '@fortawesome/fontawesome';
 import fontawesomeSolid from '@fortawesome/fontawesome-free-solid';

src/app/utils/polyfills.utils.ts

@@ -0,0 +1,10 @@
+// tslint:disable-next-line:interface-name
+interface StringConstructor {
+  isEmpty(str: string | undefined): boolean;
+  emptyIfUndefined(str: string): string;
+}
+
+// tslint:disable-next-line:no-null-keyword
+String.isEmpty = str => str === undefined || str === null || str.length === 0;
+
+String.emptyIfUndefined = str => String.isEmpty(str) ? '' : str;