Fix anchors erroring when heading is empty

[Rendez]

Motivation

Found a bug because one of our Markdown docs contained an empty heading

# Heading 1
##

Notice the empty Heading 2, makes the anchors `toSlug()` crash because of the internal `toString()` usage

Here's part of the stacktrace:

TypeError: Cannot read property 'toString' of undefined
    at module.exports (/[...]/node_modules/docusaurus/lib/core/toSlug.js:41:6)
    at Object.md.renderer.rules.heading_open (/[...]/node_modules/docusaurus/lib/core/anchors.js:16:20)
    at Renderer.render (/[...]/node_modules/remarkable/lib/renderer.js:71:39)
    at Remarkable.render (/[...]/node_modules/remarkable/lib/index.js:152:24)
    [...]

Have you read the Contributing Guidelines on pull requests?

Yes

Test Plan

I added a test to specifically check for these cases.

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[JoelMarcey]

@Rendez Thank you for the pull request. Can you please sign the CLA so that we can review/merge it?

https://code.facebook.com/cla

[yangshun]

Thanks for fixing this. Just have a small question regarding the test.

[yangshun]

Why does it equal <h1> instead of <h1></h1>?

[Rendez]

Because the render method references the rules.heading_open alone const render = md.renderer.rules.heading_open;. We do not run the remarkable parser, but simply invoke this one function. Am I right?

Until now, the tests' expectations though checked to see if the opening header tag contained an id attribute, etc., so the new test's expectation looks unusual.

[yangshun]

Verified that the test results are intended. Thanks @Rendez!