docs: Improve note on allows unassigned

docs/src/modules/ROOT/pages/responding-to-change/responding-to-change.adoc

@@ -93,7 +93,34 @@ To implement this:
 . Add a score level (usually a medium level between the hard and soft level) by switching xref:constraints-and-score/overview.adoc#scoreType[`Score` type].
 . Make the planning variable xref:using-timefold-solver/modeling-planning-problems.adoc#planningVariableAllowingUnassigned[allow unassigned values].
 . Add a score constraint on the new level (usually a medium constraint) to penalize the number of unassigned entities (or a weighted sum of them).
+Use xref:constraints-and-score/score-calculation.adoc#constraintStreamsForEach[`forEachIncludingUnassigned`] and check if the xref:using-timefold-solver/modeling-planning-problems.adoc#planningVariable[planning variable] or xref:using-timefold-solver/modeling-planning-problems.adoc#bidirectionalVariable[inverse relation shadow variable] is `null`:
 
+[tabs]
+====
+Java::
++
+[source,java,options="nowrap"]
+----
+    private Constraint penalizeUnassignedVisits(ConstraintFactory factory) {
+        return factory.forEachIncludingUnassigned(Visit.class)
+                .filter(visit -> visit.getVehicle() == null)
+                .penalize(HardMediumSoftScore.ONE_MEDIUM)
+                .asConstraint("Unassigned Visit");
+    }
+----
+
+Python::
++
+[source,python,options="nowrap"]
+----
+def penalize_unassigned_visits(factory: ConstraintFactory) -> Constraint:
+    return (factory.for_each_including_unassigned(Visit)
+                   .filter(lambda visit: visit.vehicle is None)
+                   .penalize(HardMediumSoftScore.ONE_MEDIUM)
+                   .as_constraint("Unassigned Visit")
+    )
+----
+====
 
 [#overconstrainedPlanningWithVirtualValues]
 === Overconstrained planning with virtual values

docs/src/modules/ROOT/pages/using-timefold-solver/modeling-planning-problems.adoc

@@ -488,10 +488,13 @@ Use xref:constraints-and-score/score-calculation.adoc#constraintStreamsForEach[f
 Timefold Solver will automatically add the value `null` to the value range.
 There is no need to add `null` in a collection provided by a ``ValueRangeProvider``.
 
-[NOTE]
+[IMPORTANT]
 ====
 Using a planning variable with unassigned values implies
 that your score calculation is responsible for punishing (or even rewarding) variables with a `null` value.
+
+Failure to penalize unassigned variables can cause a solution with *all* variables unassigned to be the best solution.
+See the xref:responding-to-change/responding-to-change.adoc#overconstrainedPlanningWithNullValues[overconstrained planning with `null` variable values] section in the docs for more infomation.
 ====
 
 [WARNING]
@@ -1676,12 +1679,15 @@ customers: Annotated[list[Customer], PlanningListVariable(allows_unassigned_valu
 ----
 ====
 
-[NOTE]
+[IMPORTANT]
 ====
 Constraint Streams filter out unassigned planning values by default.
 Use xref:constraints-and-score/score-calculation.adoc#constraintStreamsForEach[forEachIncludingUnassigned()] to avoid such unwanted behaviour.
 Using a planning list variable with unassigned values implies
 that your score calculation is responsible for punishing (or even rewarding) these unassigned values.
+
+Failure to penalize unassigned values can cause a solution with *all* values unassigned to be the best solution.
+See the xref:responding-to-change/responding-to-change.adoc#overconstrainedPlanningWithNullValues[overconstrained planning with `null` variable values] section in the docs for more infomation.
 ====
 
 xref:responding-to-change/responding-to-change.adoc[Repeated planning]