Make use-package-normalize-value handle nil better

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.