message: fix: do not show discriminator dropdown if it is empty
diff: <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>
  