RFR: Filter out invalid GitLab merge requests

[Robin Westberg]

Sometimes GitLab returns merge request data that doesn't contain a head hash. Since we can't really do anything with these, just ignore them.

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Commit messages:
 - Filter out invalid MRs

Changes: https://git.openjdk.java.net/skara/pull/956/files
 Webrev: https://webrevs.openjdk.java.net/?repo=skara&pr=956&range=00
  Stats: 12 lines in 1 file changed: 12 ins; 0 del; 0 mod
  Patch: https://git.openjdk.java.net/skara/pull/956.diff
  Fetch: git fetch https://git.openjdk.java.net/skara pull/956/head:pull/956

PR: https://git.openjdk.java.net/skara/pull/956