Languages labels cause schema error when not set

Problem/Motivation

The dropdown_language block cause the following schema error "variable type is NULL but applied schema class is Drupal\Core\Config\Schema\Sequence" when the 'labels' value is NULL.

Steps to reproduce

- Navigate to /admin/config/regional/dropdown-language-switcher
- In "Display Language Labelling" select any value but "Custom Labels"
- In Block Layout, add the Dropdown Language block and save it
- The value of the settings.labels will be null because the block form only displays the labels form element for "Custom Labels" config
- This causes schema error

Proposed resolution

This is fixed by marking the labels as nullable in the schema