Handle formatting output of empty maps and lists

[emh-jump]

Adds a special case when a list or map is empty

Fixes #817

Added 0 as a non-magic number since a constant would likely make this code, and other code in the repo like it, more confusing. Can revert and add a constant if folks would really prefer.

[prembhaskal]

looks good, but i don't have write access, please ping in carvel slack to get some traction.