RFR: forge: patch for commit can be null

[Erik Helin]

Hi all,

please review this patch that handles cases where the GitHub REST API returns `null` for the `"patch"` field for a `commit` object. This seems to primarily happen for very large merges that changes 200+ files.

Testing:
- [x] Manual testing on Linux x64

Thanks,
Erik

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Commit messages:
 - forge: patch for commit can be null

Changes: https://git.openjdk.java.net/skara/pull/914/files
 Webrev: https://webrevs.openjdk.java.net/?repo=skara&pr=914&range=00
  Stats: 5 lines in 1 file changed: 3 ins; 0 del; 2 mod
  Patch: https://git.openjdk.java.net/skara/pull/914.diff
  Fetch: git fetch https://git.openjdk.java.net/skara pull/914/head:pull/914

PR: https://git.openjdk.java.net/skara/pull/914