Allow empty string for certifierId

Author: callmekatootie

src/modules/achievement/service.js

@@ -12,15 +12,15 @@ const methods = helper.getServiceMethods(
     achievementsProviderId: joi.string().required(),
     name: joi.string().required(),
     uri: joi.string(),
-    certifierId: joi.string().allow(),
+    certifierId: joi.string().allow(''),
     certifiedDate: joi.date().format('iso')
   },
   { // patch request body joi schema
     userId: joi.string().required(),
     achievementsProviderId: joi.string().required(),
     name: joi.string(),
     uri: joi.string(),
-    certifierId: joi.string().allow(),
+    certifierId: joi.string().allow(''),
     certifiedDate: joi.date().format('iso')
   },
   { // search request query joi schema