Conditionally hide the PDF link in comparison pages

When no PDF is provided, better hide the button instead of it being a dead link.

I tested this in https://gitlab.com/gitlab-com/www-gitlab-com/merge_requests/5031, so you can review how this looks by the review app in that MR.

merged

mentioned in commit 2c75083f

Looks good. @axil I see someone added PDFs for Trello and Jenkins. Has someone else been generating them?

@SeanPackham, @iamphill @marcia and @connorshea are the only ones I know! Basically, anyone that has a mac.

@SeanPackham yeah I think that was me, I updated them about a week ago.