Fix exception on null body.

[ehuss]

I'm not sure what changed, but my feed stopped working because it was crashing
due to an issue without a body. The body is null and marked was failing
with this error:

  Error: marked(): input parameter is undefined or null
      at marked (/gh-feed/node_modules/marked/lib/marked.js:1244:11)
      at JSON.parse.slice.forEach.issue (/gh-feed/index.js:155:18)
      at Array.forEach (<anonymous>)
      at Object.genFeed (/gh-feed/index.js:148:4)
      at genFeed.next (<anonymous>)
      at Object.dispatch (/gh-feed/node_modules/koa-router/lib/router.js:334:5)
      at dispatch.next (<anonymous>)
      at Object.compress (/gh-feed/node_modules/koa-compress/index.js:42:5)
      at compress.next (<anonymous>)
      at Object.<anonymous> (/gh-feed/node_modules/koa-compose/index.js:28:5)