Fix $language default parameter

[janschoenherr]

This PR fixes the default value for the $language parameter. It makes it equal to Joomla\CMS\Helper\RouteHelper found in libraries/src/Helper/RouteHelper.php.

I am not sure if this is documented somewhere and that documentation would need to be updated.

[chmst]

looks good to me but do you have a test instruction? Had you an error message or warning here?

[janschoenherr]

There is no error, it is just phpstan reporting this.

[ceford]

I have tested this item ✅ successfully on 73a9741

Looked at the code and tested the patch. I doubt this would ever show up as a problem. No documentation change required.

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[Quy]

I have tested this item ✅ successfully on 73a9741

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[Quy]

RTC

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[wilsonge]

Thanks!