Empty image shouldn't cause error

[shushugah]

Currently an empty string value for image frontmatter (can happen when deleting image in DecapCMS) causes HTMLProofer to spit an error

• See example commit fix 9c0e2ba
• See example Netlify build error https://app.netlify.com/sites/techworkersco/deploys/67e495c386c9d80008fbf75e

Solutions

• Make the event code more resiliant for empty strings
• or customize https://github.com/gjtorikian/html-proofer which is called in Rakefile https://github.com/techworkersco/twc-site/blob/master/Rakefile to ignore such cases