Stock and sale price empty values should be preserved.
1st pass
update
Fix PHP Notice.
Use wpsc_product_variation_price_from() instead of wpsc_product_variation_price_available()
Fix compatibility issues with the references to the wpsc-theme folder

- WPSC_THEME_ENGINE_V1_PATH and WPSC_THEME_ENGINE_V1_URL should be used to refer to the templates' paths.
- Restore the wpsc-theme folder, just in case old themes are still relying on these files being there.