Check exports.frontmatter before creating javascriptFrontmatter nodes

[cwgw]

Closes #4095.

This breaks the using-javascript-transforms example, which actually expected a js file without frontmatter to be included with allJavascriptFrontmatter during page creation.

I've included a simple fix for that as well—adding frontmatter to that file—but now I wonder if this is a more significant change than I'd previously thought.

That said, I still think it makes the most sense that the javascript frontmatter plugin only create nodes from files that actually export frontmatter.

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[m-allanson]

It sounds like the behaviour you've added is more correct than the old behaviour, but creates a breaking change? @jbolda @KyleAMathews what's the best thing to do here? Can we give gatsby-transformer-javascript-frontmatter a major version bump?

[KyleAMathews]

Since the plugin is brand new, I tend to be ok with doing what's technically breaking changes. You could think of this as a bug fix. It shouldn't actually break any code since people would have had to add JS code filtering out empty query results — that could would just cease to filter anything not break.

[KyleAMathews]

Thanks @cwgw!

[cwgw]

Thank you for suggesting that I give it a shot. 👍

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