Issue #3280: fix fail on shipping step

[korovitskyi]

Description (*)

Added condition to avoid validation not existing form.

Fixed Issues (if relevant)

1. Fixes magento/inventory#3280

Manual testing scenarios (*)

Configure "The In-Store delivery" method.

1. Geolocation: a.)install geolocation bin/magento inventory-geonames:import us (if USA only); b) set configuration Store → Configuration → Catalog → Inventory → Distance Provider for Distance Based SSA → Provider: Offline Calculation.
2. Stock: a) Add two or more sources at Store → Source → Add New Source. Important: set real American Latitude and Longitude. I searched it on google maps. b) Create stock and add to it our sources Store → Source → Add New Stock. Set Main Website to the sales channels.
3. Enable The In-Store delivery shipping method.
4. Go to the product, there is added source tab: a) remove the default source. b) Assign already created sources. c) Add qty and set available stock status.
5. Add this product(one) to the cart and go on checkout.

Questions or comments

Contribution checklist (*)

• [ ] Pull request has a meaningful description of its purpose
• [ ] All commits are accompanied by meaningful commit messages
• [ ] All new or changed code is covered with unit/integration tests (if applicable)
• [ ] All automated tests passed successfully (all builds are green)

[korovitskyi]

@magento run all tests