Make use-package-normalize-value handle nil better

The previous version of `use-package-normalize-value', when passed
nil, would return the list (symbol-value (quote nil)). This meant that
keywords which used `use-package-normalize-value' or the higher-level
normalizer `use-package-normalize-test' would get a non-nil
argument (i.e. the above list) even when the user specified nil to the
package.

This had the concrete impact of making it so that :defer-install nil
was treated as :defer-install t.

lisp/use-package/use-package.el

@@ -778,7 +778,8 @@ If the package is installed, its entry is removed from
 
 (defsubst use-package-normalize-value (label arg)
   "Normalize a value."
-  (cond ((symbolp arg)
+  (cond ((null arg) nil)
+        ((symbolp arg)
          `(symbol-value ',arg))
         ((functionp arg)
          `(funcall #',arg))