Prevent users from assigning an invalid issue type to new issues

What does this MR do?

Prior this change, when creating a new issue, the passed parameter issue_type wasn't validated for its presence. We only checked if the corresponding policy is allowed.

For example:

• ✔ When passing issue_type=incident we check if create_incident policy exists.

• ✅ When passing issue_type=foo the policy check failed because there was no create_foo policy.

• 💥 When passing issue_type=project, however, passes the policy check for create_project but the following assignment of issue_type in Issue fails with ArgumentError: 'project' is not a valid issue_type and results with 500 error.

📓 issue_type will be replaced by WorkItem::Type soon see !66519 (merged)).

This commit checks the presence of the passed issue_type before checking the corresponding policy to prevent such error.

Related issue

I've noticed this bug while working on #336624 (closed) and decided to fix it right away given the simple fix. Hence Stuff that should Just Work.

Screenshots or Screencasts (strongly suggested)

Example: Passing issue_type=project manually as in http://localhost:3000/gitlab-org/gitlab-shell/-/issues/new?issuable_template=incident&issue[issue_type]=project:

Before	After

How to setup and validate locally (strongly suggested)

1. Start GDK
2. Open http://localhost:3000/gitlab-org/gitlab-shell/-/issues/new?issuable_template=incident&issue[issue_type]=project
3. See "new issue" form where "Type" is selected to "Issue"

Does this MR meet the acceptance criteria?

Conformity

• [-] I have included changelog trailers, or none are needed. (Does this MR need a changelog?)
  • It's a small bugfixes which users usually do not stumble upon
• [-] I have added/updated documentation, or it's not needed. (Is documentation required?)
• I have properly separated EE content from FOSS, or this MR is FOSS only. (Where should EE code go?)
• [-] I have added information for database reviewers in the MR description, or it's not needed. (Does this MR have database related changes?)
• I have self-reviewed this MR per code review guidelines.
• This MR does not harm performance, or I have asked a reviewer to help assess the performance impact. (Merge request performance guidelines)
• I have followed the style guides.
• [-] This change is backwards compatible across updates, or this does not apply.

Availability and Testing

• I have added/updated tests following the Testing Guide, or it's not needed. (Consider all test levels. See the Test Planning Process.)
• I have tested this MR in all supported browsers, or it's not needed.
• [-] I have informed the Infrastructure department of a default or new setting change per definition of done, or it's not needed.

Security

Does this MR contain changes to processing or storing of credentials or tokens, authorization and authentication methods or other items described in the security review guidelines? If not, then delete this Security section.

• [-] Label as security and @ mention @gitlab-com/gl-security/appsec
• [-] The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• [-] Security reports checked/validated by a reviewer from the AppSec team