RFR: Avoid trying to update build number for tags that don't have one

[Robin Westberg]

The issue notifier can be configured to update the build number of issues when a corresponding tag is created. However, certain tags don't contain build number information, so don't attempt this for those.

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Commit messages:
 - Avoid trying to update build number for tags that don't have one

Changes: https://git.openjdk.java.net/skara/pull/1082/files
 Webrev: https://webrevs.openjdk.java.net/?repo=skara&pr=1082&range=00
  Stats: 31 lines in 5 files changed: 6 ins; 0 del; 25 mod
  Patch: https://git.openjdk.java.net/skara/pull/1082.diff
  Fetch: git fetch https://git.openjdk.java.net/skara pull/1082/head:pull/1082

PR: https://git.openjdk.java.net/skara/pull/1082