Env var set to empty in shell does not override .env file

Created by: geritol

Is this a bug report?

yes

Environment

System: OS: macOS 10.14 CPU: x64 Intel(R) Core(TM) i5-5257U CPU @ 2.70GHz Binaries: Node: 8.12.0 - /usr/local/bin/node npm: 6.4.1 - /usr/local/bin/npm Browsers: Chrome: 70.0.3538.102 Firefox: 63.0 Safari: 12.0 npmPackages: react: ^16.6.3 => 16.6.3 react-dom: ^16.6.3 => 16.6.3 react-scripts: 2.1.1 => 2.1.1 npmGlobalPackages: create-react-app: Not Found

Steps to Reproduce

(Write your steps here:)

1. put REACT_APP_VAR=test in .env.production
2. put console.log("VAR", process.env.REACT_APP_VAR); inApp.js
3. run REACT_APP_VAR= npm run build
4. serve the app (serve -s build)

Expected Behavior

The console should containt: VAR

Actual Behavior

The console contains VAR test

Reproducible Demo

https://github.com/geritol/cra-env-repro

Similar to #4002 (closed) but the issue here is that you cannot override with an empty value the vars provided in .env files.