Check for null preview in WidgetItem.hasGeneratedPreview

AppWidgetManager.getWidgetPreview has been updated to return null
if the provider does not exist or the caller does not have permission to
access its previews. This change updates WidgetItem to check for that.

Bug: 308041327
Test: GeneratedPreviewTest#widgetItem_hasGeneratedPreview_nullPreview
Flag: ACONFIG com.android.launcher3.enable_generated_previews TRUNKFOOD
Change-Id: Ic2489c8698a81ba1bac005b2eb77348bcbfab999