RFR: 2563: NPE when reviewers id changed due to census update

[Zhao Song]

When Skara Bot processes the backport command, it attempts to parse reviewers IDs from the original commit message. It then looks up these reviewers IDs in the current repository census to get the reviewers' full names, and then put the full names in the pull request description for backport.

A corner case may occur if the repository census is updated between the original commit and the backport creation. In this case, the reviewer ID in the original commit may be from a previous census, but later the bot could lookup the ID in the updated census. If the reviewer ID is no longer present in the updated census, this can result in a NPE.

We should fallback to use the id instead of full name.

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Commit messages:
 - SKARA-2563

Changes: https://git.openjdk.org/skara/pull/1733/files
  Webrev: https://webrevs.openjdk.org/?repo=skara&pr=1733&range=00
  Issue: https://bugs.openjdk.org/browse/SKARA-2563
  Stats: 9 lines in 1 file changed: 4 ins; 0 del; 5 mod
  Patch: https://git.openjdk.org/skara/pull/1733.diff
  Fetch: git fetch https://git.openjdk.org/skara.git pull/1733/head:pull/1733

PR: https://git.openjdk.org/skara/pull/1733