RFR: 1455: Github closed action may not contain an actor

[Erik Joelsson]

As described in the bug, pr.closedBy() may (consistently) return empty on Github in some cases. Rather than getting stuck retrying endlessly, I think we should just add a fallback.

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Commit messages:
 - SKARA-1455

Changes: https://git.openjdk.java.net/skara/pull/1323/files
 Webrev: https://webrevs.openjdk.java.net/?repo=skara&pr=1323&range=00
  Issue: https://bugs.openjdk.java.net/browse/SKARA-1455
  Stats: 1 line in 1 file changed: 0 ins; 0 del; 1 mod
  Patch: https://git.openjdk.java.net/skara/pull/1323.diff
  Fetch: git fetch https://git.openjdk.java.net/skara pull/1323/head:pull/1323

PR: https://git.openjdk.java.net/skara/pull/1323