Draft: Short circuit vulns resolver when unavailable

What does this MR do and why?

This change modifies Resolvers::VulnerabilitiesResolver to short circuit with an empty result if the :security_dashboard feature is unavailable.

This fixes an edge case where GQL endpoint returns hasNextPage: true in the pagination data, but returns an empty result set.

This can occur on a project that previously had this licensed feature available, and has vulnerabilities, but the licensed feature is now turned off. This happens because the resolver loads the vulnerabilities, but they are all subsequently redacted. The check for hasNextPage happens independently of the redaction.

How to set up and validate locally

Your installation will need to have some vulnerabilities present.

Apply the following patch to simulate the :security_dashboard licensed feature being unavailable

diff --git a/ee/app/models/license.rb b/ee/app/models/license.rb
index 94b48382ea09..90287c5e99ea 100644
--- a/ee/app/models/license.rb
+++ b/ee/app/models/license.rb
@@ -75,6 +75,8 @@ def block_changes?
     def feature_available?(feature)
       # Include features available per plan + usage ping features if Usage Pings is enabled
       # as instance setting.
+      return false if feature == :security_dashboard
+
       !!current&.feature_available?(feature) ||
         GitlabSubscriptions::Features.usage_ping_feature?(feature)
     end

1. Run this GraphQL query on master

{
  vulnerabilities(first: 10) {
    nodes {
      id
      title
    }
    pageInfo {
      endCursor
      hasNextPage
    }
  }
}

2. You should see the following result, note result set is empty but hasNextPage is true

{
  "data": {
    "vulnerabilities": {
      "nodes": [],
      "pageInfo": {
        "endCursor": null,
        "hasNextPage": true
      }
    }
  }
}

3. Switch to this MR branch
4. Repeat the query above, you should see the following result with hasNextPage: false

{
  "data": {
    "vulnerabilities": {
      "nodes": [],
      "pageInfo": {
        "endCursor": null,
        "hasNextPage": false
      }
    }
  }
}

Related to #433513