RFR: 2247: When pr's headHash is missing, stop further processing

[Zhao Song]

Kevin Rushforth found this GitLab issue few years ago.

Sometimes, pull requests in GitLab would be stuck. When it happens, it shows the number of commits as "0". Clicking on the "Changes" tab shows no diffs, and displays a warning banner: "Something went wrong on our end. Please try again!" This requires action on user's part to get them "unstuck".

In Skara side, when it happens, pull request bot would not be able to get the headHash of this pull request and then throw an exception and trigger an other round of CheckWorkItem. If the user doesn't fix it, skara bot will continuously work on the pull request.

To resolve it, we should try to make pull request bot be able to identify this situation and provide some guidance to the users.

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Commit messages:
 - SKARA-2247

Changes: https://git.openjdk.org/skara/pull/1645/files
  Webrev: https://webrevs.openjdk.org/?repo=skara&pr=1645&range=00
  Issue: https://bugs.openjdk.org/browse/SKARA-2247
  Stats: 13 lines in 1 file changed: 13 ins; 0 del; 0 mod
  Patch: https://git.openjdk.org/skara/pull/1645.diff
  Fetch: git fetch https://git.openjdk.org/skara.git pull/1645/head:pull/1645

PR: https://git.openjdk.org/skara/pull/1645