Convert Option<Vec<sort expression>> to Vec<sort expression>

[findepi]

Consider AggregateFunction.order_by field -- it is Option<Vec< /*sort expr*/ >>
None means "no sorting" but this can be modeled as Some( vec![] ).
Empty collection perfectly describes lack of sorting, so no need to wrap if with optional value.

Remove Option and keep just Vec.

This can be applied to

• AggregateFunction.order_by
• ExprFuncBuilder.order_by
• ...

based on #12178 (comment) cc @crepererum

[findepi]

take

[findepi]

plan to work on this after #12193, as the two will conflict.