Fix warning in PayPal Express when DB is not setup

Provided a default value of an empty array to get_option() in the
PayPal Express merchant.
Merge pull request #1043 from jbeales/fix-paypal-express-array

Fix warning in PayPal Express when DB is not setup
Don't show DB upgrade notice for new DB

Remove database & theme upgrade notices if WPEC hasn’t installed its
database yet, (the database install notice is displayed separately).
Merge branch 'show-only-install-notice' of https://github.com/jbeales/WP-e-Commerce into show-only-install-notice