Publicize: Avoid stripping empty lines from sharing description

I'm not sure if there's anything we can do about this or not, but particularly when sharing to Facebook, I noticed that empty lines are automatically stripped out. I feel like this was a recent change (within the past six months) so perhaps Facebook changed something.

#### Steps to reproduce the issue

1. Start a new post.
2. Enter some text in the sharing field for Publicize. Include one empty line between paragraphs.
3. Publish post.
4. The empty line will be stripped out.

Here's a screenshot example:

It's totally possible this is a restriction from Facebook, but I thought it was worth mentioning because it's an annoyance I run into every week (and manually correct on Facebook).

No file, test, or code entry point is identified. Start by tracing the Publicize sharing flow for Facebook and compare the submitted description with the entered text, then determine whether the empty line is removed by Jetpack or Facebook. Done means establishing the source of the stripping and documenting or fixing the behavior accordingly.

Written by the indexing model from the issue text.