Fix 'list-tags' when invoked from a non-file buffer

Fix 'list-tags' when invoked from a non-file buffer

This use case was broken by the improvement that attempts to
offer the current buffer's file name as the default file whose
tags to list.
* lisp/progmodes/etags.el
(tags--get-current-buffer-name-in-tags-file): Doc fix.  Return nil
if no file is associated with the current buffer, and avoid
signaling an error if 'buffer-file-name' returns nil.  (Bug#37611)
(list-tags): Doc fix.  Signal an error if the user specifies no
file name at the prompt.

* doc/emacs/maintaining.texi (List Identifiers): Fix wording of
the documentation of 'list-tags'.