check-minimum-order API Show understanding message when cart is empty.

[dheerajqa-webkul]

Summary:
when cart is empty and hit check-minimum-order show "Attempt to read property "items" on null error.

Step to reproduce:

• Customer login
• Remove Cart product

Expected Result :
Show proper & understand Message

Actual Result :
Show Error "Attempt to read property "items" on null

Screenshot:
https://github.com/user-attachments/assets/02221c83-ab0b-46ed-a43b-b745d99339f9