[3.x] Fix export var inferring wrong type of preload

[ArshPanesar]

When assigning 'export var' to preload, the class_name for export property was not bieng set in the _parse_class() function, which caused the export's native type to be mistaken as Object when using _type_from_property() function:

_type_from_property(PropertyInfo p_property, ...)
{ 
        DataType ret;
        ...
        // class_name is not set, so native_type = "Object"
        ret.native_type = p_property.class_name == StringName() ? "Object" : p_property.class_name;
        ...
        return ret;
}

This caused a parse error when used in a script.

Setting class_name for export property in _parse_class() removed the parse error.

Fixes #61752