修改关于 NULL 的问题

Oracle 中  原始sql  ：alter table aaa modify bbb null;
转换后成为：  ALTER TABLE  aaa  MODIFY ( bbb DEFAULT NULL );
原始Sql的语义改变了，改变了字段的default值

src/main/java/com/alibaba/druid/sql/ast/statement/NullConstraint.java

@@ -0,0 +1,30 @@
+/*
+ * Copyright 1999-2101 Alibaba Group Holding Ltd.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+package com.alibaba.druid.sql.ast.statement;
+
+import com.alibaba.druid.sql.visitor.SQLASTVisitor;
+
+public class NullConstraint extends SQLConstraintImpl implements SQLColumnConstraint {
+
+    public NullConstraint(){
+    }
+
+    @Override
+    protected void accept0(SQLASTVisitor visitor) {
+        visitor.visit(this);
+        visitor.endVisit(this);
+    }
+}