get NPE when calling Azure.webApps().list()

https://github.com/Azure/azure-sdk-for-java/issues/2003

sdk version:

stacktrace:

```
java.lang.NullPointerException
at com.microsoft.azure.management.appservice.implementation.WebAppsImpl$1.filter(WebAppsImpl.java:47)
at com.microsoft.azure.management.appservice.implementation.WebAppsImpl$1.filter(WebAppsImpl.java:38)
at com.microsoft.azure.management.resources.fluentcore.utils.PagedListConverter.convert(PagedListConverter.java:65)
at com.microsoft.azure.management.appservice.implementation.WebAppsImpl.wrapList(WebAppsImpl.java:91)
at com.microsoft.azure.management.resources.fluentcore.arm.collection.implementation.TopLevelModifiableResourcesImpl.list(TopLevelModifiableResourcesImpl.java:118)
at com.microsoft.azuretools.core.mvp.model.webapp.AzureWebAppMvpModel.listWebAppsBySubscriptionId(AzureWebAppMvpModel.java:335)
at com.microsoft.azuretools.core.mvp.model.webapp.AzureWebAppMvpModel.listWebApps(AzureWebAppMvpModel.java:354)
at com.microsoft.tooling.msservices.serviceexplorer.azure.webapp.WebAppModulePresenter.onModuleRefresh(WebAppModulePresenter.java:32)
at com.microsoft.tooling.msservices.serviceexplorer.azure.webapp.WebAppModule.refreshItems(WebAppModule.java:35)
at com.microsoft.tooling.msservices.serviceexplorer.RefreshableNode.refreshItems(RefreshableNode.java:92)
at com.microsoft.tooling.msservices.serviceexplorer.RefreshableNode$2.run(RefreshableNode.java:130)
at com.microsoft.azuretools.core.azureexplorer.helpers.IDEHelperImpl$1.run(IDEHelperImpl.java:55)
at org.eclipse.core.internal.jobs.Worker.run(Worker.java:56)
```

The subscription used is in MoonCake.