fix(popover): fix undefined container class (#2283)

Author: IlyaSurmay

src/popover/popover.directive.ts

@@ -37,7 +37,7 @@ export class PopoverDirective implements OnInit, OnDestroy {
   /**
    * Css class for popover container
    */
-  @Input() public containerClass: string;
+  @Input() public containerClass: string = '';
 
   /**
    * Returns whether or not the popover is currently being shown