Editor: Cease adding paragraphs when switching editor tabs.

Fixes an error since [41783] in which extra paragraphs could be added to content when switching editor tabs.

Props pento, adamsilverstein, aduth.
Fixes #42531.


Built from https://develop.svn.wordpress.org/trunk@42177


git-svn-id: https://core.svn.wordpress.org/trunk@42007 1a063a9b-81f0-0310-95a4-ce76da25c4cd