UploadPack: Prevent setting null protocolV2Hook

The setProtocolV2Hook sets the protocolV2Hook to whatever value is
passed, which could be null, but the invocations of protocolV2Hook's
methods are not guarded by null-checks.

Annotate the parameter as @Nullable and set ProtocolV2Hook.DEFAULT
when null is passed. This makes the implementation consistent with
other similar methods that set a hook or filter with possible null
value.

Change-Id: I70919a3248d4c2658783941a37c47e437cff0baa
Signed-off-by: David Pursehouse <david.pursehouse@gmail.com>

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

@@ -583,10 +583,11 @@ public class UploadPack {
 	 * Set the protocol V2 hook.
 	 *
 	 * @param hook
+	 *            the hook; if null no special actions are taken.
 	 * @since 5.1
 	 */
-	public void setProtocolV2Hook(ProtocolV2Hook hook) {
+	public void setProtocolV2Hook(@Nullable ProtocolV2Hook hook) {
-		this.protocolV2Hook = hook;
+		this.protocolV2Hook = hook != null ? hook : ProtocolV2Hook.DEFAULT;
 	}
 
 	/**