Feature request: Enable prefixing keys not present on build time.

[LuisMesquita]

Hi, thank you for the work on building this package, super helpful.

My current company follows the Build once, deploy many philosophy, and your package is invaluable. But we faced one issue. During build time during our CI pipeline, we don't have any .env file, as this is injected later through Kubernetes. This causes the makePublicVar to skip our later-on inject variables.

I know there are ways to work around this, like passing a list on env when running the yarn build, TO_BE_INJECT_ENV= yarn build, or having a dump .env file on the CI environment. But this goes against our guidelines.

So I have opened this PR with a proposal to add a new makePublicVar option to skip or not empty variables. Let me know what you think.