fix(laravel): correct cart emptiness check in ProductController

[sentry]

Fixes FRONTEND-REACT-6BV. The issue was that: Backend's checkout method throws an exception due to an undefined variable check during inventory validation, resulting in a 500 error and frontend explicitly throwing on undefined response status.

• Moved the empty($quantities) check to occur after the $quantities array has been populated from $cart['quantities'].
• Ensures that the cart emptiness validation accurately reflects the contents of the processed quantities.
• Prevents an incorrect "Invalid checkout request: cart is empty" exception when the cart actually contains items.

This fix was generated by Seer in Sentry, triggered by Isaac Wang. 👁️ Run ID: 11848668

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1 Skipped Deployment

Project	Deployment	Actions	Updated (UTC)
empower	Ignored	Preview	Mar 21, 2026 7:18pm

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 22.01%. Comparing base (6aab80f) to head (4f15831).
⚠️ Report is 2 commits behind head on master.
✅ All tests successful. No failed tests found.

❌ Your project check has failed because the head coverage (22.01%) is below the target coverage (35.00%). You can increase the head coverage or adjust the target coverage.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1374      +/-   ##
==========================================
- Coverage   22.02%   22.01%   -0.02%     
==========================================
  Files          42       42              
  Lines        1362     1363       +1     
  Branches      176      176              
==========================================
  Hits          300      300              
- Misses       1035     1037       +2     
+ Partials       27       26       -1     

Flag	Coverage Δ
api	5.82% <ø> (-0.02%)	⬇️
frontend	32.37% <ø> (ø)

Components	Coverage Δ
checkout_module	5.82% <ø> (-0.02%)	⬇️
product_component	26.03% <ø> (ø)
see 2 files with indirect coverage changes