Fix tmm-menubar when 'tmm-completion-prompt' is nil

Fix tmm-menubar when 'tmm-completion-prompt' is nil

* lisp/tmm.el (tmm-prompt): Handle nil value of 'tmm-mid-prompt'.
(tmm-completion-delete-prompt): Don't rely on the exact text of
the completion heading line, as it is now a customizable format
string, and can be nil, meaning no heading line is inserted at
all.  Instead, search for the first character of the menu based on
text properties used for it.  (Bug#63754)