check for null

Author: Vladimir Kotal

test/org/opensolaris/opengrok/history/RepositoryFactoryTest.java

@@ -62,7 +62,9 @@ private void testNotWorkingRepository(String repoPath, String propName)
         String origPropValue = System.setProperty(propName, "/foo/bar/nonexistent");
         File root = new File(repository.getSourceRoot(), repoPath);
         Repository repo = RepositoryFactory.getRepository(root);
-        System.setProperty(propName, origPropValue);
+        if (origPropValue != null) {
+            System.setProperty(propName, origPropValue);
+        }
         assertFalse(repo.isWorking());
     }