AND Items formatter; Invalid render array key for 1 item, extra "and" for additional items.

Steps to reproduce:

Have a webform containing checkboxes with 2 options
Set it's Item format to "value" and Items format to "and"

# 1:
Submit the webform, but select only one option.
View the submission in the results tab on the webform. This triggers the following error:

User error: "0" is an invalid render array key in Drupal\Core\Render\Element::children()

#2
Submit the same webform, but select both option.
View the submission in the results tab on the webform. You'll see an extra "and" after the second value:

[value1] and [value2] and.