magento2 theme processing fix for 3786

[akosglue]

• Bug fix #…? Magento 2: no assets generated when using custom magento_themes without split_static_deployment #3786
• New feature?
• BC breaks?
• Tests added?
• Docs added?

with the following configuration in deploy.php

set('static_content_jobs', '16');
set('static_content_locales', 'nl_BE fr_BE');
set('split_static_deployment', false);

it produces the following output for these cases (when running magento:deploy:assets):

set('magento_themes', []);
-->
.../current/bin/magento setup:static-content:deploy -f --content-version=1713786990  nl_BE fr_BE  -j 16

set('magento_themes', ['Magento/luma', 'Custom/another']);
-->
.../current/bin/magento setup:static-content:deploy -f --content-version=1713787032  nl_BE fr_BE  -t Magento/luma -t Custom/another -j 16

set('magento_themes', [
    'Magento/blank'   => 'nl_BE',
    'Magento/luma'   => null,
    'myvendor/theme-at' => 'de_AT',
    'myvendor/theme-be' => 'fr_BE nl_BE',
    'Custom/another' => '{{static_content_locales}} it_IT',
]);
-->
.../current/bin/magento setup:static-content:deploy -f --content-version=1713787073  nl_BE fr_BE  -t Magento/blank -t Magento/luma -t myvendor/theme-at -t myvendor/theme-be -t Custom/another -j 16

note that the languages are not taken from the associative array but from the 'static_content_locales' parameter, but this is only when 'split_static_deployment' is false