You may update without changing the code. This happens when I test a plugin with a new version of WordPress and don’t need to make any code changes.
Update the readme.txt with new info, then:
svn ci -m "My cool message"This is the same command to push new changes that do involve updated code.
If SVN is unauthenticated use your wordpress.org account username (which is not the email address) and password. For me that is mikehealy.