Handle optional fields

[zvan]

I'm working on a project and found your plugin. My MDX fields are optional in Netlify CMS.

I'm changing Gatsbys schema with

...
body: String @mdx
...

that resulted in build error whenever the field was undefined

Error: TypeError [ERR_INVALID_ARG_TYPE]: The "data" argument must be of type string or an instance of Buffer,
   TypedArray, or DataView. Received undefined

I managed to fix build errors with returning null.

[zslabs]

Thanks!