Pdf generation breaks on the new Optional template blocks

[jeromesimeon]

Describe the bug
Pdf generation fails when applied to a CiceroMark containing Optional nodes.

To Reproduce
Steps to reproduce the behavior:

1. Open the attached zip, containing a sample/template and corresponding data and CiceroMark
2. Call the transform with: markus transform --from ciceromark_parsed --to pdf --input ciceromark_parsed.json
   3.See error:

bash-3.2$ markus transform --from ciceromark_parsed --to pdf --input cm.json 
(node:19761) UnhandledPromiseRejectionWarning: Error: Unhandled type Optional
    at ToPdfMakeVisitor.visit (/Users/jeromesimeon/.nvm/versions/node/v10.22.0/lib/node_modules/@accordproject/markdown-cli/node_modules/@accordproject/markdown-pdf/lib/ToPdfMakeVisitor.js:302:15)
    at ValidatedConcept.accept (/Users/jeromesimeon/.nvm/versions/node/v10.22.0/lib/node_modules/@accordproject/markdown-cli/node_modules/@accordproject/concerto-core/lib/model/typed.js:57:24)
    at nodes.forEach.node (/Users/jeromesimeon/.nvm/versions/node/v10.22.0/lib/node_modules/@accordproject/markdown-cli/node_modules/@accordproject/markdown-pdf/lib/ToPdfMakeVisitor.js:143:12)
    at Array.forEach (<anonymous>)

Expected behavior
Properly renders to pdf.

Attached zip

optionals.zip

[jeromesimeon]

Fixed in #296