RFR: Filter out invalid GitLab merge requests [v3]

[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.

Robin Westberg has updated the pull request incrementally with one additional commit since the last revision:

  Make it actually compile

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Changes:
  - all: https://git.openjdk.java.net/skara/pull/956/files
  - new: https://git.openjdk.java.net/skara/pull/956/files/9c71d535..1c708407

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=skara&pr=956&range=02
 - incr: https://webrevs.openjdk.java.net/?repo=skara&pr=956&range=01-02

  Stats: 1 line in 1 file changed: 0 ins; 0 del; 1 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