Merge pull request from waymondo/master

Allow customized values to be nil
GitHub-reference: https://github.com/jwiegley/use-package/issues/515

lisp/use-package/use-package.el

@@ -1423,7 +1423,8 @@ deferred until the prefix key sequence is pressed."
             (value (nth 1 def))
             (comment (nth 2 def)))
         (when (or (not variable)
-                  (not value)
+                  (and (not value)
+                       (not (eq value nil)))
                   (> (length def) 3)
                   (and comment (not (stringp comment))))
           (use-package-error error-msg))))))