The relevant section is here: https://github.com/sbt/sbt/blob/0.13/src/sphinx/Detailed-Topics/Launcher.rst#syntax.  It doesn't say the empty string is allowed or disallowed as a default.  I'm not sure it is necessary to add it explicitly.

For the API diff, let's have one line in the changes section (Community/ChangeSummary_0.13.0) under Improvements for now.  A howto section or wherever is appropriate can be added for 0.13.1..