RFR: webrev: not all commits changes files

[Erik Helin]

Hi all,

please review this small patch that updates webrev's JSON generator to handle commits that don't change any files. This
can happen for example with trivial merges (since we use the `-c` flag to `git log`).

Thanks,
Erik

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Commit messages:
 - webrev: not all commits changes files

Changes: https://git.openjdk.java.net/skara/pull/754/files
 Webrev: https://webrevs.openjdk.java.net/?repo=skara&pr=754&range=00
  Stats: 1 line in 1 file changed: 0 ins; 0 del; 1 mod
  Patch: https://git.openjdk.java.net/skara/pull/754.diff
  Fetch: git fetch https://git.openjdk.java.net/skara pull/754/head:pull/754

PR: https://git.openjdk.java.net/skara/pull/754