[DocsUI, Search]: Fix crash on search reset.

Early reset of search can cause crash due to lateinit var mTaskList not
being yet initialized. We set it to empty value to prevent this from
happening.

Bug: 399263486
Test: m DocumentsUIGoogle
Flag: com.android.documentsui.flags.use_search_v2_rw
Change-Id: I3c1c49361251a3c4f8bbc6e0036f05059c6f22d1