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GH-41149: [C++][Acero] Fix asof join race (#41614)



### Rationale for this change

Sporadic asof join test failures have been frequently and annoyingly observed in pyarrow CI, as recorded in #40675 and #41149.

Turns out the root causes are the same - a logical race (as opposed to physical race which can be detected by sanitizers). By injecting special delay in various places in asof join, as shown in https://github.com/zanmato1984/arrow/commit/ea3b24c5f7308fe42f60dad41f51dbcbc1a54929, the issue can be reproduced almost 100%. And I have put some descriptions in that commit to explain how the race happens.

### What changes are included in this PR?

Eliminate the logical race of emptiness by combining multiple call-sites of `Empty()`.

### Are these changes tested?

Include the UT to reproduce the issue.

### Are there any user-facing changes?

None.

**This PR contains a "Critical Fix".**
In #40675 and #41149 , incorrect results are produced.
* GitHub Issue: #41149 
* Also closes #40675

Authored-by: Ruoxi Sun <zanmato1984@gmail.com>
Signed-off-by: Antoine Pitrou <antoine@python.org>