which-function-mode: put hook function on after-change-major-mode-hook

which-function-mode: put hook function on after-change-major-mode-hook

, rather than find-file-hook.  This keeps which-function-mode active should
the major mode be reinitialized.  Also accept a null result from
add-log-current-defun as definitive, should that function have run.  This
fixes bug #40714.

* lisp/progmodes/which-func.el (which-func-ff-hook): Put on
after-change-major-mode-hook.
(which-function): Enhance the logic to accept a null result from
add-log-current-defun.