[fix](Nereids) type coercion on case-when is not correct (#12650)

When we do type coercion on CaseWhen expression, such as sql like this:
```
CASE WHEN n_nationkey > 1 THEN n_regionkey ELSE 0 END
```
The ELSE part 0 need do type coercion as CAST (0 AS INT). But we miss it in PR #11802

fe/fe-core/src/main/java/org/apache/doris/nereids/trees/expressions/CaseWhen.java

@@ -65,7 +65,11 @@ public class CaseWhen extends Expression {
     }
 
     public List<DataType> dataTypesForCoercion() {
-        return whenClauses.stream().map(WhenClause::getDataType).collect(Collectors.toList());
+        List<DataType> result = whenClauses.stream().map(WhenClause::getDataType).collect(Collectors.toList());
+        if (defaultValue.isPresent()) {
+            result.add(defaultValue.get().getDataType());
+        }
+        return result;
     }
 
     public <R, C> R accept(ExpressionVisitor<R, C> visitor, C context) {