fix sql formater bug

hutool-db/src/main/java/cn/hutool/db/sql/SqlFormatter.java

@@ -1,6 +1,11 @@
 package cn.hutool.db.sql;
 
-import java.util.*;
+import cn.hutool.core.util.StrUtil;
+
+import java.util.HashSet;
+import java.util.LinkedList;
+import java.util.Set;
+import java.util.StringTokenizer;
 
 /**
  * SQL格式化器 from Hibernate
@@ -303,6 +308,9 @@ public class SqlFormatter {
 		}
 
 		private static boolean isFunctionName(String tok) {
+			if(StrUtil.isEmpty(tok)){
+				return true;
+			}
 			char begin = tok.charAt(0);
 			boolean isIdentifier = (Character.isJavaIdentifierStart(begin)) || ('"' == begin);
 			return (isIdentifier) && (!LOGICAL.contains(tok)) && (!END_CLAUSES.contains(tok)) && (!QUANTIFIERS.contains(tok)) && (!DML.contains(tok)) && (!MISC.contains(tok));

hutool-db/src/test/java/cn/hutool/db/sql/SqlFormatterTest.java

@@ -0,0 +1,14 @@
+package cn.hutool.db.sql;
+
+import org.junit.Test;
+
+public class SqlFormatterTest {
+
+	@Test
+	public void formatTest(){
+		// issue#I3XS44@Gitee
+		// 测试是否空指针错误
+		String sql = "(select 1 from dual) union all (select 1 from dual)";
+		SqlFormatter.format(sql);
+	}
+}