Fix NPE in CommitOnlyTest#getHead

Change-Id: I05abd00d151cf9834d1a097dd16dc280b62a7edd
Signed-off-by: Matthias Sohn <matthias.sohn@sap.com>

org.eclipse.jgit.test/tst/org/eclipse/jgit/api/CommitOnlyTest.java

@@ -1294,9 +1294,15 @@ public class CommitOnlyTest extends RepositoryTestCase {
 		try {
 			final Repository repo = git.getRepository();
 			final ObjectId headId = repo.resolve(Constants.HEAD + "^{commit}");
+			if (headId == null) {
+				return "";
+			}
 			try (RevWalk rw = new RevWalk(repo)) {
 				final TreeWalk tw = TreeWalk.forPath(repo, path,
 						rw.parseTree(headId));
+				if (tw == null) {
+					return "";
+				}
 				return new String(tw.getObjectReader().open(tw.getObjectId(0))
 						.getBytes());
 			}