RFR: 1091: "PR body must not be empty" check pointless for clean backports

[Erik Joelsson]

This patch removes the requirement to write something in the PR body for clean backports. Most of the time there really is nothing to add in that case.

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Commit messages:
 - SKARA-1091

Changes: https://git.openjdk.java.net/skara/pull/1201/files
 Webrev: https://webrevs.openjdk.java.net/?repo=skara&pr=1201&range=00
  Issue: https://bugs.openjdk.java.net/browse/SKARA-1091
  Stats: 23 lines in 3 files changed: 9 ins; 6 del; 8 mod
  Patch: https://git.openjdk.java.net/skara/pull/1201.diff
  Fetch: git fetch https://git.openjdk.java.net/skara pull/1201/head:pull/1201

PR: https://git.openjdk.java.net/skara/pull/1201