Null safety for TaskDescription in RemoteTaskInfo.

The taskDescription in TaskInfo can be null, resulting in an NPE. I need to go back and make this a bit more robust, but temporarily handle the null pointer for now.

Test: Added unit tests
Bug: 400970610
Flag: android.companion.enable_task_continuity
Change-Id: Idf25849920fe4b9bda5249f9d594e9a80a22f944