fix(ajsf/core): layout functions: buildTitleMap: use empty string to conform HTML behaviour

use empty string instead of null for non required empty values to conform HTML behaviour
Fixes hamzahamidi#333

Author: E-Fir

projects/ajsf-core/src/lib/shared/layout.functions.ts

@@ -1062,7 +1062,7 @@ export function buildTitleMap(
     }
   }
   if (!fieldRequired && !hasEmptyValue) {
-    newTitleMap.unshift({ name: '<em>None</em>', value: null });
+    newTitleMap.unshift({ name: '<em>None</em>', value: '' });
   }
   return newTitleMap;
 }