Fix null array offset deprecation on PHP 8.5 in Select::getRawValue()

[thedotedge]

On PHP 8.5, Select::getRawValue() triggers:

Using null as an array offset is deprecated, use an empty string instead

at HTML/QuickForm2/Element/Select.php:183. When a select's value is null (e.g. an empty/unselected option, or a datasource returning null), the loop does $this->possibleValues[$value] with $value === null, using null as an array offset.

This casts $value to string in the offset lookup. null becomes '' — which PHP already coerced it to implicitly when used as an array key — so behavior is unchanged on all supported PHP versions, and the 8.5 deprecation no longer fires. (Numeric option values are unaffected: (string) 5 and 5 resolve to the same array key.)

This deprecation is fatal under strict mode, so any form containing a <select> fails to render on PHP 8.5.