[spotbugs] Fix potential NPE in FileRepository#convertToReftable

File#listFiles can return null. Use Files#list which does not return
null and should be faster since it's returning directory entries lazily
while File#listFiles fetches them eagerly.

Change-Id: I3bfe2a52278244fc469143692c06b05d9af0d0d4
Signed-off-by: Matthias Sohn <matthias.sohn@sap.com>

org.eclipse.jgit/src/org/eclipse/jgit/internal/storage/file/FileRepository.java

@@ -21,6 +21,7 @@ import java.io.FileNotFoundException;
 import java.io.FileOutputStream;
 import java.io.IOException;
 import java.io.OutputStream;
+import java.nio.file.Files;
 import java.text.MessageFormat;
 import java.text.ParseException;
 import java.util.ArrayList;
@@ -727,7 +728,8 @@ public class FileRepository extends Repository {
 			throws IOException {
 		File reftableDir = new File(getDirectory(), Constants.REFTABLE);
 		File headFile = new File(getDirectory(), Constants.HEAD);
-		if (reftableDir.exists() && reftableDir.listFiles().length > 0) {
+		if (reftableDir.exists()
+				&& Files.list(reftableDir.toPath()).findAny().isPresent()) {
 			throw new IOException(JGitText.get().reftableDirExists);
 		}