[DocsUI, Search]: Fix work profile crash.

When switching to work profile while using DocumentsUI as a file picker
it is possible for the top of the folder stack to return null. This CL
adds protection against this case. If the top of the stack is null, we
resort to listing the root. A unit test that checks the new code works
as intended is added to FolderLoaderTest.

Bug: 417872920
Test: DocumentsUIUnitTests
Flag: com.android.documentsui.flags.use_search_v2_read_only
Change-Id: I034d2a0c876c232b431d3b5d6217c59b550655d1