SVYX-926 TiNG Typehead issue when NULL has a displayValue

made the null value starts with SVY_

Author: jcompagner

projects/bootstrapcomponents/src/typeahead/typeahead.ts

@@ -13,7 +13,7 @@ import { ServoyBootstrapBasefield } from '../bts_basefield';
 })
 export class ServoyBootstrapTypeahead extends ServoyBootstrapBasefield<HTMLInputElement> implements IPopupSupportComponent{
 
-    private readonly NULL_VALUE = 'TS_NULL_VALUE';
+    private readonly NULL_VALUE = 'SVY_TS_NULL_VALUE_';
 
     @ViewChild('instance') instance: NgbTypeahead;