Linking to a parallel diff note line number should not create gray box

Linking to a parallel diff note line number should not a empty create gray box.

We should either open the discussion or simply do nothing.

ex. https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/12150/diffs?diff_id=4941055&view=parallel#dab608e1d5ee562b9ad2378e4a22cac4f0743612_0_121

As discovered by @DouweM, https://gitlab.slack.com/archives/C0GQHHPGW/p1497984303086505