RFR: 1053: Github check in conclusion "stale" not handled

[Erik Joelsson]

This patch adds handling of the "stale" github check result. A stale result does not have a completed_at time, so we need to handle that this field can be null.

I tried to look through the code to see if there were any places where we would need to react to this new result, but could only find one that seemed relevant.

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Commit messages:
 - SKARA-1053

Changes: https://git.openjdk.java.net/skara/pull/1172/files
 Webrev: https://webrevs.openjdk.java.net/?repo=skara&pr=1172&range=00
  Issue: https://bugs.openjdk.java.net/browse/SKARA-1053
  Stats: 13 lines in 4 files changed: 11 ins; 0 del; 2 mod
  Patch: https://git.openjdk.java.net/skara/pull/1172.diff
  Fetch: git fetch https://git.openjdk.java.net/skara pull/1172/head:pull/1172

PR: https://git.openjdk.java.net/skara/pull/1172