fix issue in gconv.Struct* functions panic when converting attribute value is nil

.example/net/ghttp/server/session/basic/session.go

@@ -0,0 +1,25 @@
+package main
+
+import (
+	"github.com/gogf/gf/frame/g"
+	"github.com/gogf/gf/net/ghttp"
+	"github.com/gogf/gf/os/gtime"
+)
+
+func main() {
+	s := g.Server()
+	s.Group("/", func(group *ghttp.RouterGroup) {
+		group.GET("/set", func(r *ghttp.Request) {
+			r.Session.Set("time", gtime.Timestamp())
+			r.Response.Write("ok")
+		})
+		group.GET("/get", func(r *ghttp.Request) {
+			r.Response.WriteJson(r.Session.Map())
+		})
+		group.GET("/clear", func(r *ghttp.Request) {
+			r.Session.Clear()
+		})
+	})
+	s.SetPort(8199)
+	s.Run()
+}