Fix persist with missing body from file (regression from #759)

[ebello]

- Summary
Trying to save a page that does not have a value for body results in an error. This is a regression from #759, which removed the default value for body.

- Test plan
If you try to save the Kitchen Sink entry you'll end up with the following error:

Uncaught TypeError: Cannot read property 'content' of undefined
    at module.exports (stringify.js?2ea5:23)
    at Function.matter.stringify (index.js?1c62:131)
    at Object.toFile (frontmatter.js?cfa9:57)
    at Backend.entryToRaw (backend.js?bb12:303)
    at Backend.persistEntry (backend.js?bb12:239)
    at eval (entries.js?c1a9:300)
    at eval (index.js?8bff:9)
    at eval (bindActionCreators.js?45eb:3)
    at eval (EntryPage.js?2e7b:109)

Or run new should stringify YAML with missing body test in frontmatter.spec.js.

- Description for the changelog
Includes a failing test as well as initiates body with an empty string if not available from page data.

- A picture of a cute animal (not mandatory but encouraged)

[tech4him1]

LGTM. We were actually discussing this on the chat right now, so it's awesome timing!

[tech4him1]

@ebello Can you add yourself to our contributors list? Instructions here: https://github.com/netlify/netlify-cms/blob/master/CONTRIBUTING.md#add-yourself-as-a-contributor

[erquhart]

Oh, I thought this fix was from you @tech4him1 - thanks for this @ebello!! Definitely add yourself to the contributors list!

[ebello]

Thanks guys! I just did so, hopefully I did it correctly.