Don't let an authorization be empty

It create problems with some servers like ProGet when used as a nuget
cache.

It's most likely the source of yarnpkg#825 and yarnpkg#1260

src/registries/npm-registry.js

@@ -177,7 +177,7 @@ export default class NpmRegistry extends Registry {
       }
     }
 
-    return '';
+    return undefined;
   }
 
   getScopedOption(scope: string, option: string): mixed {