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GH-39690: [C++][FlightRPC] Fix nullptr dereference in PollInfo (#39711)



### Rationale for this change

The current implementation is a bit painful to use due to the lack of a move constructor.

### What changes are included in this PR?

- Fix a crash in PollInfo with a nullptr FlightInfo.
- Declare all necessary constructors (https://en.cppreference.com/w/cpp/language/rule_of_three)

### Are these changes tested?

Yes.

### Are there any user-facing changes?

Yes, this adds new copy constructors.

* Closes: #39673.
* Closes: #39690

Authored-by: David Li <li.davidm96@gmail.com>
Signed-off-by: David Li <li.davidm96@gmail.com>