Handle empty markdown (or without title)

[aardvarksaurus]

If you let empty the Markdown text field, or if you fill it so the conversion to html does not give any child node (prez.js, line 31; you can do this with just some spaces for example), you won't get any "prez" (😉) and you'll have in your console the following error:

TypeError: Cannot read property 'type' of undefined
    at _updatePrez (wanaprez.dist.js:44900)
    at _doPrez (wanaprez.dist.js:45002)
    at _fulfilled (wanaprez.dist.js:39090)
    at wanaprez.dist.js:39119
    at Promise.promise.promiseDispatch (wanaprez.dist.js:39052)
    at wanaprez.dist.js:38860
    at runSingle (wanaprez.dist.js:38373)
    at flush (wanaprez.dist.js:38361)
    at MessagePort.channel.port1.onmessage (wanaprez.dist.js:38455)

It could be better if you have something like a warning message beforehand (instead of an empty white screen due to the error).

Maybe you can pre-analyze the text to see if it contains at least one valid title?