Sequence translation

Problem/Motivation

During work on #2409701: Field storage configuration is not exposed to config translation UI, a bug with translation of "sequence" data types was discovered.

Exported translated configuration looked like this:

settings:
  allowed_values:
    label: string1
    1:
      label: string2

While it was expected to look like this:

settings:
  allowed_values:
    -
      label: string1
    -
      label: string2

Bug source

In \Drupal\config_translation\FormElement\ListElement::setConfig(), the following string creates the element ID appending the element key to the base key (which may be not provided, so NULL).

$element_key = implode('.', array_filter(array($base_key, $key)));

The problem is that for sequences, the key could be "0", so it's filtered by the array_filter() function.

Proposed resolution

Change the code to:

$element_key = ltrim("$base_key.$key", '.');

Remaining tasks

+ create patch
- write tests

User interface changes

none

API changes

none