RFR: 2468: PR bot mistakenly stop warning about empty pr body

[Zhao Song]

In a pull request, the user didn't put anything in the pr body. At first, skara bot successfully detected the empty body and showed the error "The pull request body must not be empty." However, after the pull request is updated, the error disappeared.
After investigation, I found that if the pr body is empty, skara bot would add a "EMPTY_PR_BODY_MARKER" to the pr body, and later, skara bot would mistakenly treat it as the body added by user.

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Commit messages:
 - SKARA-2468

Changes: https://git.openjdk.org/skara/pull/1712/files
  Webrev: https://webrevs.openjdk.org/?repo=skara&pr=1712&range=00
  Issue: https://bugs.openjdk.org/browse/SKARA-2468
  Stats: 2 lines in 1 file changed: 1 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/skara/pull/1712.diff
  Fetch: git fetch https://git.openjdk.org/skara.git pull/1712/head:pull/1712

PR: https://git.openjdk.org/skara/pull/1712