Render empty blocks

[joeha480]

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.

[joeha480]

This shadows some input conversion problems, that should probably be made visible.

[bertfrees]

Maybe it's easier to implement if you disallow empty blocks? Just an idea.

[bertfrees]

Related: https://github.com/brailleapps/dotify/issues/150

[bertfrees]

I added the "OBFL" label because we first have to fix this on the specification level. This includes the other issues that are related to empty blocks, namely #48 and braillespecs/obfl#74.