RFR: Fix NPE in toString

Mon Aug 8 21:21:04 UTC 2022

My patch in SKARA-1530 did unfortunately trigger NPE in the toString() methods of all subclasses of PullRequestWorkItem. This is because the pr field isn't initialized in the constructor anymore. The repository is available through the bot field however.

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Commit messages:
 - Fix NPE in toString

Changes: https://git.openjdk.org/skara/pull/1351/files
 Webrev: https://webrevs.openjdk.org/?repo=skara&pr=1351&range=00
  Stats: 3 lines in 3 files changed: 0 ins; 0 del; 3 mod
  Patch: https://git.openjdk.org/skara/pull/1351.diff
  Fetch: git fetch https://git.openjdk.org/skara pull/1351/head:pull/1351

PR: https://git.openjdk.org/skara/pull/1351