Update env.js without requirement for NODE_ENV, fix #10195

packages/react-scripts/config/env.js

@@ -16,20 +16,15 @@ const paths = require('./paths');
 delete require.cache[require.resolve('./paths')];
 
 const NODE_ENV = process.env.NODE_ENV;
-if (!NODE_ENV) {
-  throw new Error(
-    'The NODE_ENV environment variable is required but was not specified.'
-  );
-}
 
 // https://github.com/bkeepers/dotenv#what-other-env-files-can-i-use
 const dotenvFiles = [
-  `${paths.dotenv}.${NODE_ENV}.local`,
+  NODE_ENV && `${paths.dotenv}.${NODE_ENV}.local`,
   // Don't include `.env.local` for `test` environment
   // since normally you expect tests to produce the same
   // results for everyone
   NODE_ENV !== 'test' && `${paths.dotenv}.local`,
-  `${paths.dotenv}.${NODE_ENV}`,
+  NODE_ENV && `${paths.dotenv}.${NODE_ENV}`,
   paths.dotenv,
 ].filter(Boolean);