Custom provider fixes (#4498)

Fixes: https://github.com/transloadit/uppy/issues/4460

examples/custom-provider/server/CustomProvider.cjs

@@ -34,7 +34,9 @@ function adaptData (res) {
 class MyCustomProvider {
   static version = 2
 
-  authProvider = 'myunsplash'
+  static get authProvider () {
+    return 'myunsplash'
+  }
 
   // eslint-disable-next-line class-methods-use-this
   async list ({ token, directory }) {

packages/@uppy/companion/src/server/provider/credentials.js

@@ -77,7 +77,7 @@ exports.getCredentialsOverrideMiddleware = (providers, companionOptions) => {
       return
     }
 
-    if (!companionOptions.providerOptions[providerName].credentialsURL) {
+    if (!companionOptions.providerOptions[providerName]?.credentialsURL) {
       next()
       return
     }