Description

If you have a adoc without a title, say:

test

You'll get a cryptic error:

      Cannot read property 'getCombined' of undefined

This looks like an issue in gatsby-transformer-asciidoc/src/gatsby-node.js L91:

      document: {
        title: title.getCombined(),
        subtitle: title.hasSubtitle() ? title.getSubtitle() : ``,
        main: title.getMain(),
      },

where there's no check if title is undefined or null.

Steps to reproduce

(shown above)

Expected result

Either (a) things should work or (b) some sensible error like "You must specify a title" is shown

Actual result

      Cannot read property 'getCombined' of undefined

Environment

System:
OS: macOS 10.15.7
CPU: (12) x64 Intel(R) Core(TM) i7-8850H CPU @ 2.60GHz
Shell: 5.7.1 - /bin/zsh
Binaries:
Node: 12.19.0 - ~/.nvm/versions/node/v12.19.0/bin/node
Yarn: 1.22.0 - /usr/local/bin/yarn
npm: 6.14.8 - ~/.nvm/versions/node/v12.19.0/bin/npm
Languages:
Python: 2.7.16 - /usr/bin/python
Browsers:
Chrome: 86.0.4240.111
Firefox: 81.0.2
Safari: 14.0
npmPackages:
gatsby: ^2.24.85 => 2.24.85
gatsby-plugin-react-head: ^1.1.0 => 1.1.0
gatsby-plugin-sass: ^2.3.22 => 2.3.22
gatsby-source-filesystem: ^2.3.37 => 2.3.37
gatsby-transformer-asciidoc: ^1.3.15 => 1.3.15
npmGlobalPackages:
gatsby-cli: 2.12.113