Allow empty material entry (lessons with titles only)

[hroncok]

There are empty material entries on pyladies.cz, but i couldn't find a way to do it here. If I provide no URL, no lesson attribute, it's invalid Material:

  - title: MicroPython a malé počítače
    type: lesson

ValueError: Unknown material type: {'type': 'lesson', 'title': 'MicroPython a malé počítače'}

I had to use this as a workaround:

  - title: MicroPython a malé počítače
    type: lesson
    url: '#'

[encukou]

@messa, Is this particular lesson beginners/micropython, or something different?

Even if it is, the issue is valid: we'll need a solution at least for Turnaj v piškvorkách in Brno.

[hroncok]

Several in http://naucse.python.cz/2017/pyladies-praha-podzim-ntk/sessions/final/

[hroncok]

• Relační databáze (SQLite)
• Speciální světelná show
• MicroPython a malé počítače

[encukou]

Those aren't materials, but ideas about what could be added to the course. See the last section in the PyLadies plan

I'd rather keep them at pyladies.cz than add them to the last lesson on naucse.

[hroncok]

Ok, I'll dig them out.

[hroncok]

#208