Fix(www): Containter component may have class: 'undefined'.

[emtei]

Set className default value to be an empty string, so undefined won't appear in HTML code if className isn't passed in props.

[pieh]

Is this (class="undefined") happening anywhere on gatsbyjs.org right now?

[emtei]

@pieh yes, i spotted it here:
https://www.gatsbyjs.org/docs/schema-generation/

[pieh]

Thanks @emtei!

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