Refactor: Move empty cart check earlier in checkout process

[seer-by-sentry]

Fixes ANDROID-19. The issue was that: Flask's checkout function attempts to use quantities before its definition, causing a NameError masked by generic exception handling.

Move the check for an empty cart to before the inventory dictionary is created. This avoids unnecessary processing when the cart is empty.

This fix was generated by Seer in Sentry, triggered by prithvirajkumar.rajakumar@sentry.io. 👁️ Run ID: 1021202

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[sentry]

🔍 Existing Issues For Review

Your pull request is modifying functions with the following pre-existing issues:

📄 File: flask/src/main.py

Function	Unhandled Issue
checkout	Exception: Error validating enough inventory for product ... Event Count: 110k
checkout	DatabaseConnectionError: get_inventory checkout Event Count: 84

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