LastConflict is not state-of-the-art

Not sure if this is actually a problem, but if I refer to Chapter 9.4.2 of the [following book](https://cse.unl.edu/~choueiry/Documents/Lecoutre-eBook-Constraint%20Networks.pdf) (p. 418), the version of LastConflict currently in the code is slightly incorrect.

The patch is, instead of this:
```java
V curDecVar = (V) decisionPath.getLastDecision().getDecisionVariable();
```
do this:
```java
Decision curDec = decisionPath.getLastDecision();
if (!curDec.hasNext() && curDec.getArity() > 1) return;
V curDecVar = curDec.getDecisionVariable();
```

I tested the two versions on XCSP3 instances:
- 'base' : current version
- 'sota': state of the art version

There is no clear winner.
We could consider a new option… or not.

Start by locating the LastConflict implementation and the decisionPath.getLastDecision() entry point, then compare its behavior with Chapter 9.4.2 of the linked book. Validate the current and proposed variants against the mentioned XCSP3 instances; done means the algorithmic choice and whether a new option is needed are resolved.

Written by the indexing model from the issue text.