v13n: Avoid potential error when the user selects verticalized standard themes in onboarding flow

[arthur791004]

Details

We need to avoid showing the error screen when the user without selected vertical selects the verticalized standard themes in the Design Picker step

See p1662043572868369-slack-C02FMH4G8

Checklist

• Avoid sending vertical id with empty string to theme-setup endpoint
• Add e2e test cases for the build flow with/without selected vertical

Related

• Design Picker: Enable standard-theme-v13n flag #67077
• Revert "Design Picker: Enable standard-theme-v13n flag" #67274