Even when there's no :config, run any pre/post config hooks

Even when there's no :config, run any pre/post config hooks

i.e., following the existing docs for use-package-inject-hooks, these hooks are
run:

use-package--foo--pre-config-hook
use-package--foo--post-config-hook

This should make config customisations more predictable (for example, spacemacs
uses these hooks extensively to allow 'layers' to be customised).

I got rid of the "special" default value for :config, because it doesn't seem to
be treated any differently than nil.

Fixes https://github.com/jwiegley/use-package/issues/785

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