As a developer, I want to specify a default-language on command  ibexa:migrations:kaliop:bulk-convert

Steps to reproduce:

1. Create a new file in migrations/test.yaml with content:

-
    type: content_type
    mode: create
    content_type_group: Test
    identifier: test
    name:
        eng-GB: 'Test'
        ger-DE: 'Test'
    description:
        eng-GB: ''
        ger-DE: ''
    name_pattern: '<title>'
    url_name_pattern: ''
    is_container: false
    attributes:
        -
            identifier: title
            type: ezstring
            name:
                eng-GB: 'Title'
                ger-DE: 'Titel'
            description:
                eng-GB: ''
                ger-DE: ''
            required: true
            searchable: true
            info-collector: false
            disable-translation: false
            category: content
            position: 10
            field-settings: {  }
            validator-configuration:
                StringLengthValidator: { maxStringLength: null, minStringLength: null }

2. run

bin/console ibexa:migrations:kaliop:bulk-convert --default-language=eng-GB migrations/ results/

expected: file should be converted in the results directory.
current: exception:

Converting file "migrations/test.yaml " failed. Failed denormalizing Step 0. Notice: Undefined index: lang