[DocsUI] add a Kotlin not-null operator to test

Prior to this commit, running atest would complain: "warning: Java type
mismatch: inferred type is 'kotlin.String?', but 'kotlin.String' was
expected".

The warning-inducing code was introduced in
I606b4be35492e0a77e6c97c6bdf514ed80cbcde8

The compiler cannot prove that the taskId string cannot be null despite
the STACK_LIST_REGEX being hard-coded.

Bug: 435032907
Flag: EXEMPT test only
Test: atest DocumentsUIGoogleTests:com.android.documentsui.ContextMenuUiTest
Change-Id: I326253cd6d68b633762f3bff2f67017ac9091fa3