Add logic to convert any null or NA formats to "" for haven.

[elimillera]

This closes #69.

Idea here is to remove any null or NA formats and convert them to "". This is due to a limitation in haven that will break any NA 'sas.formats'

[bms63]

Awesome! Thanks Eli! Can we put some stuff into the changelog for this and other issues. I feel like this is more of a minor release as we are really just fixing bugs and lingering issues we left from last year.

[elimillera]

@bms63 Should be good!

[bms63]

Love it!! Thanks Eli