scoreboard_add returns current criterion and null

docs/scarpet/api/Scoreboard.md

@@ -11,7 +11,7 @@ With specified `objective` and
 ### `scoreboard_add(objective, criterion?)`
 
 Adds a new objective to scoreboard. If `criterion` is not specified, assumes `'dummy'`.
-If the objective already exists, changes the criterion of that objective and returns false.
+If the objective already exists, changes the criterion of that objective and returns `false`. If the criterion was not specified but the objective already exists, returns the current criterion.
 If the objective was added, returns `true`. If nothing is affected, returns `null`
 
 <pre>

src/main/java/carpet/script/api/Scoreboards.java

@@ -137,6 +137,8 @@ public class Scoreboards {
 
             ScoreboardObjective objective = scoreboard.getObjective(objectiveName);
             if (objective != null) {
+                if(lv.size() == 1) return (_c, _t) -> StringValue.of(objective.getCriterion().getName());
+                if(objective.getCriterion().equals(criterion) || lv.size() == 1) return LazyValue.NULL;
                 ((Scoreboard_scarpetMixin)scoreboard).getObjectivesByCriterion().get(objective.getCriterion()).remove(objective);
                 ((ScoreboardObjective_scarpetMixin) objective).setCriterion(criterion);
                 (((Scoreboard_scarpetMixin)scoreboard).getObjectivesByCriterion().computeIfAbsent(criterion, (criterion1) -> Lists.newArrayList())).add(objective);