Avoid NPE when screen title is null

The screen title of Settings pages could be empty, e.g., Mobile network
detail page. It will cause NPE when Settings generates the slice
object, and then results in the Settings Search showing empty view for
slice search result. It’s a bad user experience, we need to add the null
check to avoid it.

Bug: 157986273
Test: robotests
Change-Id: I8bf6a081c6b3e97ccb173424ff4a310ae863ae2f