Guard against empty NODE_APP_INSTANCE

app/user_config.js

@@ -8,7 +8,8 @@ const config = require('./config');
 
 let storageProfile;
 const { NODE_ENV: environment, NODE_APP_INSTANCE:instance } = process.env;
-const isProduction = environment === 'production' && !instance;
+const isValidInstance = instance && instance.length > 0;
+const isProduction = environment === 'production' && !isValidInstance;
 
 // Use seperate data directories for each different environment and app instances
 // We should prioritise config values first
@@ -16,7 +17,7 @@ if (config.has(storageProfile)) {
   storageProfile = config.get('storageProfile');
 } else if (!isProduction) {
   storageProfile = environment;
-  if (instance) {
+  if (isValidInstance) {
     storageProfile = storageProfile.concat(`-${instance}`)
   }
 }