fix(radio): default value for falsy values

Author: SisIvanova

projects/igniteui-angular/src/lib/radio/radio.component.ts

@@ -366,6 +366,10 @@ export class IgxRadioComponent implements ControlValueAccessor, EditorProvider {
     public select() {
         this.nativeRadio.nativeElement.focus();
 
+        if (this.value === undefined || this.value === '' || this.value === null) {
+            this.value = 'on';
+        }
+
         if(!this.checked) {
             this.checked = true;
             this.change.emit({ value: this.value, radio: this });