Render empty blocks #49
Description
Don't ignore empty OBFL-blocks. A block in the source file that ends without adding any text or starting a nested block should render an empty line.
Note that lots of empty block objects (not the same as OBFL-blocks) are created by the formatter. This is because the OBFL-block hierarchy is flattened in the formatter and each transition in the hierarchy results in a new block object (because it might have different properties). These empty blocks could perhaps be merged unless they have different properties, but that's another story.