Description

Steps to reproduce:

1. Edit a page.
2. Add a <blockquote> tag with two paragraphs in it, with empty line between.
3. Save.

Actual result:
The HTML contains one paragraph.

Expected result:
The HTML contains two paragraph.

Additional information:
I did some tests over at https://www.mediawiki.org/w/index.php?title=User:Sebastian_Berlin_(WMSE)/T309778&diff=5248625&oldid=5248456

The paragraph wrapping is unintuitive, but it probably can't be changed to avoid breaking existing content that accidentally relies on it.