Add config validation to the "label" config schema data type

Problem/Motivation

Virtually every configuration entity has a label. There is no validation on this at all. There is a lot of freedom in what this string contains, but one thing is certain: it should not be empty.

Proposed resolution

Add validation constraints to

# Human readable string that must be plain text and editable with a text field.
label:
  type: string
  label: 'Label'
  translatable: true

Remaining tasks

• Failing tests
• Validation ⇒ passing tests

User interface changes

None.

API changes

None.

Data model changes

None.

Release notes snippet

TBD