[findBugs] Fix potential NPE in DirCacheCheckoutTest

Change-Id: I842cbdd14bf56cb2bd599255fe90a5a3aea0d304
Signed-off-by: Matthias Sohn <matthias.sohn@sap.com>

org.eclipse.jgit.test/tst/org/eclipse/jgit/lib/DirCacheCheckoutTest.java

@@ -1705,7 +1705,8 @@ public class DirCacheCheckoutTest extends RepositoryTestCase {
 							+ " in workDir. ", buffer, i.get(path).getBytes());
 					nrFiles++;
 				} else if (file.isDirectory()) {
-					if (file.list().length == 0) {
+					String[] files = file.list();
+					if (files != null && files.length == 0) {
 						assertEquals("found unexpected empty folder for path "
 								+ path + " in workDir. ", "/", i.get(path));
 						nrFiles++;