[opt](nereids)print numNulls of column stats in memo plan (#40029)

## Proposed changes

Issue Number: close #xxx

<!--Describe your changes.-->

fe/fe-core/src/main/java/org/apache/doris/statistics/ColumnStatistic.java

@@ -269,8 +269,9 @@ public boolean enclosed(ColumnStatistic other) {
 
     @Override
     public String toString() {
-        return isUnKnown ? "unknown" : String.format("ndv=%.4f, min=%f(%s), max=%f(%s), count=%.4f, avgSizeByte=%f",
-                ndv, minValue, minExpr, maxValue, maxExpr, count, avgSizeByte);
+        return isUnKnown ? "unknown" : String.format(
+                "ndv=%.4f, min=%f(%s), max=%f(%s), count=%.4f, avgSizeByte=%f, numNulls=%f",
+                ndv, minValue, minExpr, maxValue, maxExpr, count, avgSizeByte, numNulls);
     }
 
     public JSONObject toJson() {