修复selectList参数EntityWrapper为null时，AutoSqlInjector#sqlSelectColumns中对‘ew’的空判断

Signed-off-by: vector_ding <vector_ding@live.cn>

mybatis-plus/src/main/java/com/baomidou/mybatisplus/mapper/AutoSqlInjector.java

@@ -531,7 +531,7 @@ public class AutoSqlInjector implements ISqlInjector {
 	protected String sqlSelectColumns(TableInfo table, boolean entityWrapper) {
 		StringBuilder columns = new StringBuilder();
 		if (entityWrapper) {
-			columns.append("<choose><when test=\"ew.sqlSelect != null\">${ew.sqlSelect}</when><otherwise>");
+			columns.append("<choose><when test=\"ew != null and ew.sqlSelect != null\">${ew.sqlSelect}</when><otherwise>");
 		}
 		if (table.isKeyRelated()) {
 			columns.append(table.getKeyColumn()).append(" AS ").append(table.getKeyProperty());