RFR: 1360: SKARA-1352 is missing null check

[Magnus Ihse Bursie]

The recent fix for [SKARA-1352](https://bugs.openjdk.java.net/browse/SKARA-1352) ("Configure GithubHost as offline") is missing a null check for the configuration object.

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Commit messages:
 - 1360: SKARA-1352 is missing null check

Changes: https://git.openjdk.java.net/skara/pull/1289/files
 Webrev: https://webrevs.openjdk.java.net/?repo=skara&pr=1289&range=00
  Issue: https://bugs.openjdk.java.net/browse/SKARA-1360
  Stats: 26 lines in 1 file changed: 12 ins; 9 del; 5 mod
  Patch: https://git.openjdk.java.net/skara/pull/1289.diff
  Fetch: git fetch https://git.openjdk.java.net/skara pull/1289/head:pull/1289

PR: https://git.openjdk.java.net/skara/pull/1289