fix channel id

Author: tamizhvendan

clojure/wheel/src/wheel/marketplace/channel.clj

@@ -1,5 +1,5 @@
 (ns wheel.marketplace.channel
   (:require [clojure.spec.alpha :as s]))
 
-(s/def ::id pos-int?)
+(s/def ::id (complement clojure.string/blank?))
 (s/def ::name #{:tata-cliq :amazon :flipkart})