bug fixed for concat

src/main/java/com/alibaba/druid/sql/visitor/SQLEvalVisitorUtils.java

@@ -791,6 +791,10 @@ public class SQLEvalVisitorUtils {
         if (wallConditionContext != null) {
             wallConditionContext.setConstArithmetic(true);
         }
+        
+        if (leftValue == null || rightValue == null) {
+            return false;
+        }
 
         Object value = null;
         switch (x.getOperator()) {

src/main/java/com/alibaba/druid/sql/visitor/functions/Concat.java

@@ -33,7 +33,7 @@ public class Concat implements Function {
 
             Object itemValue = item.getAttributes().get(EVAL_VALUE);
             if (itemValue == null) {
-                continue;
+                return null;
             }
             buf.append(itemValue.toString());
         }