增加支持sql.NullString...Iterate方法。

statement.go

@@ -5,6 +5,7 @@
 package xorm
 
 import (
+	"database/sql/driver"
 	"encoding/json"
 	"errors"
 	"fmt"
@@ -509,6 +510,11 @@ func buildConditions(engine *Engine, table *core.Table, bean interface{},
 				val = engine.FormatTime(col.SQLType.Name, t)
 			} else if _, ok := reflect.New(fieldType).Interface().(core.Conversion); ok {
 				continue
+			} else if valNul, ok := fieldValue.Interface().(driver.Valuer); ok {
+				val, _ = valNul.Value()
+				if val == nil {
+					continue
+				}
 			} else {
 				engine.autoMapType(fieldValue)
 				if table, ok := engine.Tables[fieldValue.Type()]; ok {