Add debug log for empty IME list in switcher menu

This adds a debug log when we are trying to show the IME switcher menu
but we have an empty IME (and IME subtype) list, which should never be
the case. It should help understand the cause of b/325198431.

Bug: 325198431
Test: n/a
Change-Id: Ib07be176106fbc03f92f3fe99058994918fe1168