Don't fail so loudly if default work item type is invalid

What does this MR do and why?

We now raise a loud error if a default work item type is not found in the DB, explaining users that they should make sure the types are seeded into the DB.

We should not fails so loudly if the provided type is not expected to be found in the DB, even if the seeds were run correctly. Then, we return nil instead and let calling methods handle the null value as before

Fix was merge to master and deployed to .com in !153001 (merged)

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• This MR is backporting a bug fix, documentation update, or spec fix, previously merged in the default branch.
• The MR that fixed the bug on the default branch has been deployed to GitLab.com (not applicable for documentation or spec changes).
• This MR has a severity label assigned (if applicable).
• Set the milestone of the merge request to match the target backport branch version.
• This MR has been approved by a maintainer (only one approval is required).
• Ensure the e2e:package-and-test-ee job has either succeeded or been approved by a Software Engineer in Test.

Note to the merge request author and maintainer

If you have questions about the patch release process, please:

• Refer to the patch release runbook for engineers and maintainers for guidance.
• Ask questions on the #releases Slack channel (internal only).

Related to https://gitlab.com/gitlab-org/gitlab/-/issues/459904