fix(stat): 修复channel 类型错误

uni_modules/uni-stat/utssdk/common/utils/pageInfo.uts

@@ -150,7 +150,7 @@ export const get_version = () : string => {
  */
 export const get_channel = () : string => {
   const platformName = get_platform_name()
-  let channel:string|null= ''
+  let channel:string = ''
   if (platformName === 'n') {
     channel = sysAppBase.channel ?? ''
   }