Node types' `help` and `description` fields can no longer be empty strings

Previously, it was possible for a content type to have empty strings in its help and description fields; indeed, that was the default value for both of them. For example, this was allowed (node.type.import.yml):

type: import
name: Import
description: ''
help: ''
new_revision: false
display_submitted: true
preview_mode: 1
status: true
langcode: en

As of #3379091: Make NodeType config entities fully validatable, these fields cannot be empty strings; it will violate config schema. The reason is because empty strings make no sense for these fields. "" is never a useful description of a content type.

If you want them to have empty values, they need to be NULL:

type: import
name: Import
description: null
help: null
new_revision: false
display_submitted: true
preview_mode: 1
status: true
langcode: en

Note that, if these fields are NULL, \Drupal\node\Entity\NodeType::getDescription() and \Drupal\node\Entity\NodeType::getHelp() will still return empty strings, for backwards compatibility.