RFR: 1396: Gitlab MR merged outside of Skara causes NPE in mlbridge

[Erik Joelsson]

In gitlab, if an MR is merged through Gitlab, without the user of Skara, we end up in a retry loop getting NPE in the mlbridge bot. This is caused by trying to extract the value of the "closed_by" field of the PR object. This patch adds a fallback to instead look at the "merged_by" field instead.

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Commit messages:
 - SKARA-1396

Changes: https://git.openjdk.java.net/skara/pull/1298/files
 Webrev: https://webrevs.openjdk.java.net/?repo=skara&pr=1298&range=00
  Issue: https://bugs.openjdk.java.net/browse/SKARA-1396
  Stats: 11 lines in 1 file changed: 9 ins; 0 del; 2 mod
  Patch: https://git.openjdk.java.net/skara/pull/1298.diff
  Fetch: git fetch https://git.openjdk.java.net/skara pull/1298/head:pull/1298

PR: https://git.openjdk.java.net/skara/pull/1298