Cart i2: Add empty cart default inner blocks

[senadir]

Empty Cart view is currently broken in Cart i2, so we removed it. We should add it back and make sure it works the same way as Cart i1:

• Toggling it would unmount Full Cart blocks and their selection.
• The value persist (going to be hard to have to innerBlocks on the same level).

[senadir]

As of now, those points are fixed:

Toggling it would unmount Full Cart blocks and their selection.
The value persist (going to be hard to have to innerBlocks on the same level).

What's left is adding the default empty cart inner block layout.