Authorize users to read `Vulnerabilities::IssueLink` records

What does this MR do?

It is possible that a vulnerability can be related to an issue that is not accessible by the current user even though the user can access the vulnerability. In that case, we shouldn't try to serialize that issue_link otherwise the issue property of the Types::Vulnerability::IssueLinkType will be null and cause an error since the property is marked as non-nullable.

This change basically fixes the error below;

"errors": [
  {
    "message": "Cannot return null for non-nullable field VulnerabilityIssueLink.issue"
  }
]

Related to #320990 (closed), and #331308 (closed).

Does this MR meet the acceptance criteria?

Conformity

• I have included a changelog entry, or it's not needed. (Does this MR need a changelog?)
• [-] I have added/updated documentation, or it's not needed. (Is documentation required?)
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• [-] 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.

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