Textplay handling of fountain.io test cases

[arikrupnik]

I started working on section headers in Textplay. To make sure that I introduce only these changes and keep everything else constant, I tried to run Textplay on a suite of test cases from https://github.com/nyousefi/Fountain/tree/master/FountainTests

Textplay barfs on two of the test files, https://github.com/nyousefi/Fountain/blob/master/FountainTests/SectionHeaders.fountain and https://github.com/nyousefi/Fountain/blob/master/FountainTests/Synopses.fountain with the message

textplay:291:in <main>': undefined method []' for nil:NilClass (NoMethodError)

The error seens to indicate that Textplay cannot process a .fountain document without any printable content.

I wonder if this is desirable behavior, or if it's preferable for Textplay to produce a header and a footer even if there is no content.