Joomla\CMS\Installer\Installer\getParams ignores multiple=true setting

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Steps to reproduce the issue

The function getParams always initializes the values as 'string', for multiselect values this should be an array with the expanded values

[chmst]

Could you give more context?

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given for example a config field in an extension xml file like:

<field multiple="true" default="1,2" ....> <option value="1">1</option> <option value="2">2</option> <option value="3">3</option> </field> The installation will set the default value to be a string in: https://github.com/joomla/joomla-cms/blob/22f8289d039c752409854e94d3fce75caf335be6/libraries/src/Installer/Installer.php#L1746C14-L1746C56 while for a multiple=true field it should be an array (explode on , from the string).

As a result, the extension will not use the desired default values.