fix(binding): empty value error (#2169)

* fix empty value error

Here is the code that can report an error
```go
package main

import (
	"fmt"
	"github.com/gin-gonic/gin"
	"io"
	"net/http"
	"os"
	"time"
)

type header struct {
	Duration   time.Duration `header:"duration"`
	CreateTime time.Time     `header:"createTime" time_format:"unix"`
}

func needFix1() {
	g := gin.Default()
	g.GET("/", func(c *gin.Context) {
		h := header{}
		err := c.ShouldBindHeader(&h)
		if err != nil {
			c.JSON(500, fmt.Sprintf("fail:%s\n", err))
			return
		}

		c.JSON(200, h)
	})

	g.Run(":8081")
}

func needFix2() {
	g := gin.Default()
	g.GET("/", func(c *gin.Context) {
		h := header{}
		err := c.ShouldBindHeader(&h)
		if err != nil {
			c.JSON(500, fmt.Sprintf("fail:%s\n", err))
			return
		}

		c.JSON(200, h)
	})

	g.Run(":8082")
}

func sendNeedFix1() {
	// send to needFix1
	sendBadData("http://127.0.0.1:8081", "duration")
}

func sendNeedFix2() {
	// send to needFix2
	sendBadData("http://127.0.0.1:8082", "createTime")
}

func sendBadData(url, key string) {
	req, err := http.NewRequest("GET", "http://127.0.0.1:8081", nil)
	if err != nil {
		fmt.Printf("err:%s\n", err)
		return
	}

	// Only the key and no value can cause an error
	req.Header.Add(key, "")
	rsp, err := http.DefaultClient.Do(req)
	if err != nil {
		return
	}
	io.Copy(os.Stdout, rsp.Body)
	rsp.Body.Close()
}

func main() {
	go needFix1()
	go needFix2()

	time.Sleep(time.Second / 1000 * 200) // 200ms
	sendNeedFix1()
	sendNeedFix2()
}

```

* modify code

* add comment

* test(binding): use 'any' alias and require.NoError in form mapping tests

- Replace 'interface{}' with 'any' alias in bindTestData struct
- Change assert.NoError to require.NoError in TestMappingTimeUnixNano and TestMappingTimeDuration to fail fast on mapping errors

---------

Co-authored-by: Bo-Yi Wu <appleboy.tw@gmail.com>