Add form validation for AWS Nitro Enclaves

[basil]

Improve the user experience (UX) of #1066 by adding form validation.

Testing done

Manual testing of 3 cases:

• M4 (Xen, not Nitro)
• T3 (Nitro, but no Enclave)
• C6G (Nitro, with Enclave)

Submitter checklist

• [ ] Make sure you are opening from a topic/feature/bugfix branch (right side) and not your main branch!
• [ ] Ensure that the pull request title represents the desired changelog entry
• [ ] Please describe what you did
• [ ] Link to relevant issues in GitHub or Jira
• [ ] Link to relevant pull requests, esp. upstream and downstream changes
• [ ] Ensure you have provided tests - that demonstrates feature works or fixes the issue