An empty line in a mindmap currently creates a syntax error, so the following doesn't work:
@startmindmap
* Main Idea
right side
**_ Branch 1
**_ Branch 2
***_ The next branch is empty
***_
@endmindmap
www.plantuml.com/plantuml/png/SoWkIImgoStCIybDBE3IKl1DpCnJy4n9JUJYAifCpoXHACvCIUNIqefNS2fAp4lEK314vXW1EK1UI4QgGbvgHOb24aGuirWXDRUWf18Yo-MGcfS2jG40
I would've expected it to create a new blank branch, as seen here where I used a nonbreaking space as a workaround:
www.plantuml.com/plantuml/png/SoWkIImgoStCIybDBE3IKl1DpCnJy4n9JUJYAifCpoXHACvCIUNIqefNS2fAp4lEK314vXW1EK1UI4QgGbvgHOb24aGuirWXDRUWf18Ygw2cR6XcOCtba9gN0hK80000
