[player] Fix display of empty dynamic drop lists on ipad 14

The height of an empty droplist can't be calculated on ios 14
because it's in a flex container

projects/common/components/ui-elements/drop-list.component.ts

@@ -12,8 +12,11 @@ import { DragNDropValueObject, DropListElement } from '../../interfaces/elements
     <div class="list-container">
       <!-- Border width is a workaround to enable/disable the Material cdk-drop-list-receiving-->
       <!-- class style.-->
+      <!-- min-height for the following div is important for iOS 14!
+      iOS 14 is not able to determine the height of the flex container-->
       <div class="list"
            [ngClass]="{ 'align-flex' : elementModel.orientation === 'flex' }"
+           [style.min-height.px]="elementModel.position.useMinHeight ? elementModel.height - 6 : null"
            [class.dropList-highlight]="elementModel.highlightReceivingDropList"
            [style.outline-color]="elementModel.highlightReceivingDropListColor"
            [style.color]="elementModel.styling.fontColor"

projects/player/src/app/components/section/section.component.html

@@ -26,7 +26,7 @@
     <ng-container *ngFor="let element of section.elements">
       <aspect-element-splitter
           [style.min-width.px]="element.width"
-          [style.min-height.px]="element.position.useMinHeight ? element.height : 0"
+          [style.min-height.px]="element.position.useMinHeight ? element.height : null"
           [style.margin-left.px]="element.position.marginLeft"
           [style.margin-right.px]="element.position.marginRight"
           [style.margin-top.px]="element.position.marginTop"