fix: do not show discriminator dropdown if it is empty

Author: RomanHotsiy

lib/components/JsonSchema/json-schema.html

@@ -83,7 +83,7 @@
               <span *ngIf="prop.pattern" class="param-pattern">{{prop.pattern}}</span>
             </div>
             <div class="param-description" [innerHtml]="prop.description | marked"></div>
-            <div class="discriminator-info" *ngIf="prop.isDiscriminator">
+            <div class="discriminator-info" *ngIf="prop.isDiscriminator && descendants.length">
               <drop-down (change)="selectDescendantByIdx($event)" [active]="activeDescendant.idx">
                 <option *ngFor="let descendant of descendants; let i=index"
                 [value]="i" [attr.selected]="descendant.active ? '' : null" >{{descendant.name}}</option>