[DocsUI] Make listChildren test helper more robust

testRecursiveCopyDocuments_InternalStorageToDownloadsProvider is
failing because in some cases the underlying directory does not
exist and then listChildren fails to query it.

When this occurs, listChildren should be tolerant of the
DocumentsProvider.query() method returning a null Cursor.

Flag: EXEMPT bugfix
Bug: 429362203
Test: atest DocumentsUIGoogleTests:com.android.documentsui.FileCopyUiTest
Change-Id: Ia2ac193d276108efe6067494489942ac61564a95