Throws on empty markdown fields in contentful

[brandonmp]

I have an optional field on a Contentful type (with gatsby-source-contentful) that holds markdown.

When trying to run gatsby develop, nodes with an empty field break the build:

The new node didn't pass validation
ValidationError: child "internal" fails because [child "content" fails because ["content" is not allowed to be empty]]
{ id: 'c5YauqPb1uwSOy624aESCWodescriptionOverviewTextNode',
  parent: 'c5YauqPb1uwSOy624aESCWo',
  children: [],
  descriptionOverview: '',
  internal:
   { type: 'contentfulAgencyDescriptionOverviewTextNode',
     mediaType: 'text/markdown',
     content: '',
     contentDigest: 'd41d8cd98f00b204e9800998ecf8427e',
     owner: 'gatsby-source-contentful' } }

I thought the problem was from on-node-create in the source-contentful repo, but skipping createNode() when internal.content is empty doesn't fix the problem.

[KyleAMathews]

Oh weird hapijs/joi#251

I'll PR a fix for this real quick.