return null instead of an empty set.

core/src/main/java/org/keycloak/representations/idm/RealmRepresentation.java

@@ -611,12 +611,16 @@ public class RealmRepresentation {
     }
 
     public Set<String> getSupportedLocales() {
-        if(supportedLocales == null){
-            supportedLocales = new HashSet<String>();
-        }
         return supportedLocales;
     }
 
+    public void addSupportedLocales(String locale) {
+        if(supportedLocales == null){
+            supportedLocales = new HashSet<>();
+        }
+        supportedLocales.add(locale);
+    }
+
     public void setSupportedLocales(Set<String> supportedLocales) {
         this.supportedLocales = supportedLocales;
     }