Prevent ElDoc blinking when eldoc-documentation-enthusiast is used

This eldoc-documentation-strategy function didn't always obey protocol
since it returned nil sometimes, which the eldoc engine took it as a
hint for the "old" protocol to clear the echo area.

* lisp/emacs-lisp/eldoc.el (eldoc-documentation-enthusiast):
Return t.
(Version): Bump to 1.9.0

lisp/emacs-lisp/eldoc.el

@@ -5,7 +5,7 @@
 ;; Author: Noah Friedman <friedman@splode.com>
 ;; Keywords: extensions
 ;; Created: 1995-10-06
-;; Version: 1.8.0
+;; Version: 1.9.0
 ;; Package-Requires: ((emacs "26.3"))
 
 ;; This is a GNU ELPA :core package.  Avoid functionality that is not
@@ -573,7 +573,8 @@ Meant as a value for `eldoc-documentation-strategy'."
                       (let* ((callback (eldoc--make-callback :enthusiast))
                              (str (funcall f callback)))
                         (if (stringp str) (funcall callback str))
-                        nil))))
+                        nil)))
+  t)
 
 ;; JT@2020-07-10: ElDoc is pre-loaded, so in Emacs < 28 we can't
 ;; make the "old" `eldoc-documentation-function' point to the new