Use the document font size if the field font size is 0.

I've noticed sometimes the font size of a field is `0` in a pdf, which will cause the text in the text box not to be rendered.  Sure, you can pass in an explicit font size in the hash, but if you don't know the field font size in the PDF is `0`, how do you know to pass in the font size in the hash?  This can be confusing to problem to debug, so I decided it would be best to default the font size to the document's font size if:

1. there is no explicit font size in the hash
2. the font size is `0` for the field