[Hexpm] Fix badges for pre-release only versions (#10112)

* test: make existing test fail

* fix: allow null values for latest_stable_version

Author: sheerlox

services/hexpm/hexpm.service.js

@@ -14,7 +14,7 @@ const hexSchema = Joi.object({
   meta: Joi.object({
     licenses: Joi.array().required(),
   }).required(),
-  latest_stable_version: Joi.string(),
+  latest_stable_version: Joi.string().allow(null),
   latest_version: Joi.string().required(),
 }).required()
 

services/hexpm/hexpm.tester.js

@@ -50,6 +50,7 @@ t.create('version (no stable version)')
       .get('/api/packages/prima_opentelemetry_ex')
       .reply(200, {
         downloads: { all: 100 },
+        latest_stable_version: null,
         latest_version: '1.0.0-rc.3',
         meta: { licenses: ['MIT'] },
       }),