code style

mybatis-plus-core/src/main/java/com/baomidou/mybatisplus/core/conditions/AbstractWrapper.java

@@ -447,9 +447,9 @@ public abstract class AbstractWrapper<T, R, Children extends AbstractWrapper<T,
         paramNameSeq.set(0);
         paramNameValuePairs.clear();
         expression.clear();
-        lastSql.clear();
-        sqlComment.clear();
-        sqlFirst.clear();
+        lastSql.toEmpty();
+        sqlComment.toEmpty();
+        sqlFirst.toEmpty();
     }
 
     /**

mybatis-plus-core/src/main/java/com/baomidou/mybatisplus/core/conditions/SharedString.java

@@ -49,11 +49,20 @@ public class SharedString implements Serializable {
     }
 
     /**
-     * 条件清空
+     * 置 empty
      *
      * @since 3.3.1
      */
-    public void clear() {
+    public void toEmpty() {
         stringValue = StringPool.EMPTY;
     }
+
+    /**
+     * 置 null
+     *
+     * @since 3.3.1
+     */
+    public void toNull() {
+        stringValue = null;
+    }
 }

mybatis-plus-core/src/main/java/com/baomidou/mybatisplus/core/conditions/query/LambdaQueryWrapper.java

@@ -132,6 +132,6 @@ public class LambdaQueryWrapper<T> extends AbstractLambdaWrapper<T, LambdaQueryW
     @Override
     public void clear() {
         super.clear();
-        sqlSelect.setStringValue(null);
+        sqlSelect.toNull();
     }
 }

mybatis-plus-core/src/main/java/com/baomidou/mybatisplus/core/conditions/query/QueryWrapper.java

@@ -118,6 +118,6 @@ public class QueryWrapper<T> extends AbstractWrapper<T, String, QueryWrapper<T>>
     @Override
     public void clear() {
         super.clear();
-        sqlSelect.setStringValue(null);
+        sqlSelect.toNull();
     }
 }

mybatis-plus-extension/src/main/kotlin/com/baomidou/mybatisplus/extension/kotlin/KtQueryWrapper.kt

@@ -116,6 +116,6 @@ class KtQueryWrapper<T : Any> : AbstractKtWrapper<T, KtQueryWrapper<T>>, Query<K
 
     override fun clear() {
         super.clear()
-        sqlSelect.stringValue = null
+        sqlSelect.toNull()
     }
 }