Fix NRE when logged out and reading merge trains via GraphQL

What does this MR do and why?

The current check causes a null reference exception when not logged in.

This MR fixes that and adds tests for the gaps.

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Before	After

How to set up and validate locally

query {
  project(fullPath: "flightjs/flight") {
    mergeTrains {
      nodes {
        targetBranch
      }
    }
  }
}

Related to #475269 (closed)