chore: improve ConstraintAnalysis toString

Author: triceo

core/src/main/java/ai/timefold/solver/core/api/score/analysis/ConstraintAnalysis.java

@@ -61,7 +61,7 @@ public ConstraintAnalysis(@NonNull ConstraintRef constraintRef, @NonNull Score_
         }
     }
 
-    ConstraintAnalysis<Score_> negate() {
+    @NonNull ConstraintAnalysis<Score_> negate() {
         if (matches == null) {
             return new ConstraintAnalysis<>(constraintRef, weight.negate(), score.negate(), null, matchCount);
         } else {
@@ -72,9 +72,9 @@ ConstraintAnalysis<Score_> negate() {
         }
     }
 
-    static <Score_ extends Score<Score_>> ConstraintAnalysis<Score_> diff(
-            ConstraintRef constraintRef, ConstraintAnalysis<Score_> constraintAnalysis,
-            ConstraintAnalysis<Score_> otherConstraintAnalysis) {
+    static <Score_ extends Score<Score_>> @NonNull ConstraintAnalysis<Score_> diff(
+            @NonNull ConstraintRef constraintRef, @Nullable ConstraintAnalysis<Score_> constraintAnalysis,
+            @Nullable ConstraintAnalysis<Score_> otherConstraintAnalysis) {
         if (constraintAnalysis == null) {
             if (otherConstraintAnalysis == null) {
                 throw new IllegalStateException(
@@ -214,10 +214,15 @@ Explanation of score (%s):
     @Override
     public String toString() {
         if (matches == null) {
-            return "(%s at %s, no matches)"
-                    .formatted(score, weight);
+            if (matchCount == -1) {
+                return "(%s at %s, constraint matching disabled)"
+                        .formatted(score, weight);
+            } else {
+                return "(%s at %s, %d matches, justifications disabled)"
+                        .formatted(score, weight, matchCount);
+            }
         } else {
-            return "(%s at %s, %s matches)"
+            return "(%s at %s, %d matches with justifications)"
                     .formatted(score, weight, matches.size());
         }
     }