Fix return value of help--key-description-fontified

Fix return value of help--key-description-fontified

This fixes a bug with warning about obsolete commands in
`command-execute', where we incorrectly showed empty parenthesis
instead of the empty string when there was no keybinding for the new
command.

* lisp/help.el (help--key-description-fontified): Return nil instead
of the empty string if KEYS argument is nil.
* test/lisp/help-tests.el (help--key-description-fontified): New test.
(with-substitute-command-keys-test): Fix indentation.