[enhance](nereids): use optional for combine()

[jackwener]

Proposed changes

Issue Number: close #xxx

Problem Summary:

use optional for combine()

If input list is empty, combine will return true. It's strange.

Use optional to correct it.

Checklist(Required)

1. Does it affect the original behavior: (Yes/No/I Don't know)
2. Has unit tests been added: (Yes/No/No Need)
3. Has document been added or modified: (Yes/No/No Need)
4. Does it need to update dependencies: (Yes/No)
5. Are there any changes that cannot be rolled back: (Yes/No)

Further comments

If this is a relatively large or complex change, kick off the discussion at dev@doris.apache.org by explaining why you chose the solution you did and what alternatives you considered, etc...

[wangshuo128]

The current implementation looks reasonable to me.
A literal true/false predicate for empty input conjunctive/disjunctive predicates is right. It has no effect on the output data of the filter when the input predicate list is empty.
And you could check if the result is boolean literal, which has not much different from using an optional result.
A boolean literal result is much better because it has the opportunity to be optimized by boolean value rewrite rules further.

[jackwener]

I don't think so, because this function is for combine expression by use And or Or.
Empty List is an abnormal input, it's unreasonable by using LiteralTrue as result.
And LiteralTrue defeat the intent of this function. We want to combine but return a LiteralTrue, it's strange.