fix: Optional schema

[lassopicasso]

Description

Made the Schema property optional to accommodate state-less apps that may not have this configured. Having a required Schema would cause crashes in the Studio when absent.

Ref. https://github.com/Altinn/altinn-support-private/issues/3872

Related Issue(s)

• #{issue number}

Verification

• Your code builds clean without any errors or warnings
• Manual testing done (required)
• Relevant automated test added (if you find this hard, leave it and we'll help out)

Documentation

• User documentation is updated with a separate linked PR in altinn-studio-docs. (if applicable)