committing BatchUpdate with no row should complain

Running this code:

BatchUpdate update = new BatchUpdate();
update.put(key, value);
table.commit(update);

Down in getRegionServer, this triggers an NPE because the row is null (which I saw because I was running in a debugger); this NPE gets retried somewhere in the bowels of IPC. Instead, we should either remove the zero-arg BatchUpdate constructor, or have table.commit throw a runtimeexception if the row is null.