Plugin crashes when heading has no content

[davidvandusen]

When a heading has no content, the plugin crashes because it is trying to access properties of the child nodes of the heading, which don't exist.

Here is the output from a node session illustrating the issue.

$ node
> const markdownIt = require('markdown-it')
undefined
> const markdownItTocAndAnchor = require('markdown-it-toc-and-anchor').default
undefined
> markdownIt().use(markdownItTocAndAnchor).render('## Real heading\n\n##\n\nEmpty heading above')
TypeError: Cannot read property 'type' of undefined
    at Array.<anonymous> (/node_modules/markdown-it-toc-and-anchor/dist/index.js:55:53)
    at Core.process (/node_modules/markdown-it/lib/parser_core.js:51:13)
    at MarkdownIt.parse (/node_modules/markdown-it/lib/index.js:523:13)
    at MarkdownIt.render (/node_modules/markdown-it/lib/index.js:543:36)

The versions used are:

• Node.js 10.2.1
• markdown-it 8.4.1
• markdown-it-toc-and-anchor 4.1.2

[allejo]

This was introduced in #34 but still hasn't been fixed, see #34 (comment)

[medfreeman]

fixed in v4.2.0