FIO-8281: fixed sync validation error for select component with url d…

[roma-formio]

…ata src

Link to Jira Ticket

https://formio.atlassian.net/browse/FIO-8281

Description

Added 'url' data src case for sync available items check. As it's impossible to check values from 'url' data src in the sync version, just return null. I did this change because in the builder mode any url select component with default value throws validation error. It doesn't allow to save configuration for the multiple select component.

How has this PR been tested?

Use this section to describe how you tested your changes; if you haven't included automated tests, justify your reasoning

Checklist:

• I have completed the above PR template
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation (if applicable)
• My changes generate no new warnings
• My changes include tests that prove my fix is effective (or that my feature works as intended)
• New and existing unit/integration tests pass locally with my changes
• Any dependent changes have corresponding PRs that are listed above

[brendanbond]

I know this is a very very minor change, but I want us to get in the habit of adding unit/integration tests to changes like this, just so we can describe the expected behavior of the program to future developers that work on this. Please add a test case in the tests for this validation rule that describe the behavior you expect to see.

[roma-formio]

I know this is a very very minor change, but I want us to get in the habit of adding unit/integration tests to changes like this, just so we can describe the expected behavior of the program to future developers that work on this. Please add a test case in the tests for this validation rule that describe the behavior you expect to see.

@brendanbond I added test cases