Fix when a or b might be null (e.g. Async items)

[jussivesa]

Fixes Cannot read property foo of null error. For example, below code might cause error:

`<ion-item-sliding *ngFor="let item of items | async | orderBy: 'isDone' : false; let i = index;">

{{item.text}}

`

With fix proposed in pull request, no errors occur during application run.