fix(default-theme): coupon code submission no longer results in error

[niklaswolf]

Changes

closes #1325

• prevent empty coupon code submission
• fix promotion code notifications

Checklist

• the list of features is updated (if it's a feature and it's relevant)
• I followed contributing guidelines

[niklaswolf]

@mkucmus are there any valid promotion codes configured in the demo backend?
I'd need one to test also the "valid coupon code" case.
Invalid coupon code case seems to work now.

[mkucmus]

@mkucmus are there any valid promotion codes configured in the demo backend?
I'd need one to test also the "valid coupon code" case.
Invalid coupon code case seems to work now.

here's your special code: GETPROMO2021 gives 50% off 👍

[niklaswolf]

That's so generous, thanks a lot 😉😂

[vercel]

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🔍 Inspect: https://vercel.com/shopware-pwa/shopware-pwa-docs/DJUutpWU9h6fAjn2v7Y4YLxV3vVq
✅ Preview: https://shopware-pwa-d-git-fork-mothership-gmbh-1325-coupon-code-764386.vercel.app

[mkucmus]

🚀