resolves #480 don't crash on empty table (PR #511)
resolves #480 don't crash on empty table (PR #511)

- create a single empty cell if table is empty
- get number of rows from rowcount attribute
dsisnero/asciidoctor-pdf,Hextremist/asciidoctor-pdf,mojavelinux/asciidoctor-pdf,mojavelinux/asciidoctor-pdf,asciidoctor/asciidoctor-pdf,asciidoctor/asciidoctor-pdf,Hextremist/asciidoctor-pdf,dsisnero/asciidoctor-pdf