fix: don't require pricebooks

[sbellone]

The demo currently require pricebooks, while it's not an attribute that we recommend by default. Without it, it breaks with the following error:

Uncaught TypeError: data is undefined
    isSale productsPlugin.tsx:92
    ProductItem productsPlugin.tsx:152

This PR changes how the sale price is calculated, to make it work with the default attributes:

• First it looks in pricebooks
• Then using the promotionalPrice
• Then directly using the price if no other attribute is present

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SFCC-390