MissingPackageResult field in CheckPackageVersionUpdateInput made Nullable

[davidbloss]

Resolves #

Bugfix and updated struct tag examples to have actual enum values

Problem

Solution

Checklist

• I have run this code, and it appears to resolve the stated issue.
• This PR does not reduce total test coverage
• This PR has no user interface changes or has already received approval from product management to change the interface.
• Does this change require a Terraform schema change?
  • If so what is the ticket or PR #
• Make a changie entry that explains the customer facing outcome of this change

[rocktavious]

slight docstring wording change but this is MUCH better