Fix the case where :ensure is given no arguments

Fixes https://github.com/jwiegley/use-package/issues/543

lisp/use-package/use-package-ensure.el

@@ -134,7 +134,7 @@ manually updated package."
 
 (defun use-package-normalize/:ensure (name keyword args)
   (if (null args)
-      t
+      (list t)
     (use-package-only-one (symbol-name keyword) args
       #'(lambda (label arg)
           (pcase arg

test/lisp/use-package/use-package-tests.el

@@ -1469,6 +1469,14 @@
       (ignore
        (bind-keys :package mu4e ("<f9>" . mu4e))))))
 
+(ert-deftest use-package-test/543 ()
+  (match-expansion
+   (use-package hydra
+     :ensure)
+   `(progn
+      (use-package-ensure-elpa 'hydra '(t) 'nil)
+      (require 'hydra nil nil))))
+
 (ert-deftest bind-key/:prefix-map ()
   (match-expansion
    (bind-keys :prefix "<f1>"