Replace custom CSS placeholder with default value

[westonruter]

As raised by @afercia in #38667:

I was staring at the comment used in the placeholder and trying to use the keyboard to select/cancel what initially appeared to me being text... was wondering why the arrow keys weren't working at all. Maybe it's just me being a bit slow :) Since it's formatted like an inline comment /* */ it looks like actual code that can be selected. Maybe potentially confusing for users, I'd consider to remove the /* */.

[westonruter]

@melchoyce thoughts on this?

[helen]

Default value makes sense to me; may want to take the /* */ out of the translated string itself since it needs to remain a comment.

[melchoyce]

Replying to helen:

Default value makes sense to me

+1

[lukecavanagh]

@westonruter

I do not get the point of having a default value.

[westonruter]

In 39151:

Customize: Move Custom CSS control placeholder help text to setting default value.

The WP_Customize_Custom_CSS_Setting::value() method now returns the default if wp_get_custom_css() returns empty.

Props westonruter, afercia, helen.
See #35395.
Fixes #38685.

[westonruter]

Replying to lukecavanagh:

@westonruter

I do not get the point of having a default value.

The point is what @afercia raised in the description, where he expected to be able to select it. There is help text appearing in the Custom CSS textarea like this both in Jetpack and on WordPress.com.