Test if element with ID 'content' exists before attempting to retrieve its value (Fix #3395)

[aaronhipple]

Fixes JavaScript error on post types without 'editor' capability.

[Rarst]

Could you please fix up whitespace so that build passes? :) See https://travis-ci.org/Yoast/wordpress-seo/jobs/92360743#L700-L718

[aaronhipple]

Thanks @Rarst -- Saw that on Friday but it's been a little hectic for the holidays. Passing now. :)

[codeforest]

We tested and confirmed that this is working as expected now when no content div is present on the screen!

Please merge this and release it when you see fit.

Thanks @aaronhipple for contributing...

[omarreiss]

@aaronhipple thanks a lot for your contribution! The same issue was also fixed in #3363 amongst other issues. This was already merged and will be released today. I did however merge your commits into 3.0.4 branch (6eaaa88) as well to credit your contribution and make sure you will end up in the list of contributors! Thanks @codeforest for testing! I will mention you guys in the changelog as well.