fix(content): handle empty lang

[pi0]

…ighlighter

Types of changes

• Bug fix (a non-breaking change which fixes an issue)
• New feature (a non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Description

Regression from #755. Reproducable on nuxt/nuxtjs.org:

 WARN  Could not parse ./content/en/blog/moving-from-nuxtjs-dotenv-to-runtime-config.md: Cannot read property 'match' of null                                                    @nuxt/content 15:50:42

Checklist:

• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have added tests to cover my changes (if not applicable, please state why)

[benjamincanac]

@pi0 Thanks! Just released @nuxt/content v1.13.1