Add a no-op UploadPackLogger and use it by default

This matches the behavior of the hooks in UploadPack.

Change-Id: I21277e52ee9452237d87084f06bb140dfa6bf608

org.eclipse.jgit/src/org/eclipse/jgit/transport/UploadPack.java

@@ -217,7 +217,7 @@ public class UploadPack {
 
 	private PackWriter.Statistics statistics;
 
-	private UploadPackLogger logger;
+	private UploadPackLogger logger = UploadPackLogger.NULL;
 
 	/**
 	 * Create a new pack upload for an open repository.
@@ -376,6 +376,11 @@ public class UploadPack {
 		this.packConfig = pc;
 	}
 
+	/** @return the configured logger. */
+	public UploadPackLogger getLogger() {
+		return logger;
+	}
+
 	/**
 	 * Set the logger.
 	 *
@@ -1111,7 +1116,7 @@ public class UploadPack {
 		if (sideband)
 			pckOut.end();
 
-		if (logger != null && statistics != null)
+		if (statistics != null)
 			logger.onPackStatistics(statistics);
 	}
 }

org.eclipse.jgit/src/org/eclipse/jgit/transport/UploadPackLogger.java

@@ -54,6 +54,13 @@ import org.eclipse.jgit.storage.pack.PackWriter;
  * the connection information before invoking UploadPack.
  */
 public interface UploadPackLogger {
+	/** A simple no-op logger. */
+	public static final UploadPackLogger NULL = new UploadPackLogger() {
+		public void onPackStatistics(PackWriter.Statistics stats) {
+			// Do nothing.
+		}
+	};
+
 	/**
 	 * Notice to the logger after a pack has been sent.
 	 *