Widget Editor: Unable to empty widget areas in customizer

[dreamwhisper]

Description

Widget areas can't be emptied in the customizer.

Step-by-step reproduction instructions

Please list the steps needed to reproduce the bug. For example:

1. Go to the customizer.
2. Empty a widget area that already had widgets in it and click update.
3. View the front end.
4. Note the empty widget area.

Expected behaviour

Widget areas can be emptied from the customizer as they can on the widgets screen.

Actual behaviour

Widget areas can't be emptied in the customizer. The placeholder can't be removed.

Screenshots or screen recording (optional)

In Customize, only the placeholder is visible after deleting widgets. Backspacing does not remove the placeholder:

On the front end, the widget area is empty, but present in source code:

Code snippet (optional)

WordPress information

• WordPress version: 5.8-beta3-51224
• Gutenberg version: Not installed
• Are all plugins except Gutenberg deactivated? Yes
• Are you using a default theme (e.g. Twenty Twenty-One)? Yes

Device information

• Device: Mac
• Operating system: MacOS 10.15.x
• Browser: Chrome 91