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)

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• This MR is backporting a bug fix, documentation update, or spec fix, previously merged in the default branch.
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Related to #459904 (closed)