RFR: Filter out invalid GitLab merge requests [v2]

[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:

  Apply suggestions from code review
  
  Co-authored-by: Erik Duveblad <erik.helin at oracle.com>

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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/abdedc96..9c71d535

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=skara&pr=956&range=01
 - incr: https://webrevs.openjdk.java.net/?repo=skara&pr=956&range=00-01

  Stats: 9 lines in 1 file changed: 0 ins; 5 del; 4 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