TrimSqlNode should add an extra space when concatenating its contents like MixedSqlNode does

This was the [key difference](#3349 (comment)) that broke #3349 .

Author: harawata

src/main/java/org/apache/ibatis/scripting/xmltags/TrimSqlNode.java

@@ -99,6 +99,9 @@ public void applyAll() {
 
     @Override
     public void appendSql(String sql) {
+      if (!sqlBuffer.isEmpty()) {
+        sqlBuffer.append(" ");
+      }
       sqlBuffer.append(sql);
     }
 

src/test/java/org/apache/ibatis/builder/xml/dynamic/DynamicSqlSourceTest.java

@@ -214,7 +214,7 @@ void shouldTrimWHEREInsteadOfORForSecondCondition() throws Exception {
 
   @Test
   void shouldTrimWHEREInsteadOfANDForBothConditions() throws Exception {
-    final String expected = "SELECT * FROM BLOG WHERE  ID = ?   OR NAME = ?";
+    final String expected = "SELECT * FROM BLOG WHERE  ID = ?    OR NAME = ?";
     DynamicSqlSource source = createDynamicSqlSource(new TextSqlNode("SELECT * FROM BLOG"),
         new WhereSqlNode(new Configuration(),
             mixedContents(new IfSqlNode(mixedContents(new TextSqlNode("   and ID = ?   ")), "true"),
@@ -236,7 +236,7 @@ void shouldTrimNoWhereClause() throws Exception {
 
   @Test
   void shouldTrimSETInsteadOfCOMMAForBothConditions() throws Exception {
-    final String expected = "UPDATE BLOG SET ID = ?,  NAME = ?";
+    final String expected = "UPDATE BLOG SET ID = ?,   NAME = ?";
     DynamicSqlSource source = createDynamicSqlSource(new TextSqlNode("UPDATE BLOG"),
         new SetSqlNode(new Configuration(),
             mixedContents(new IfSqlNode(mixedContents(new TextSqlNode(" ID = ?, ")), "true"),