vitest 4 broke stubEnv(..., undefined) to unset an environment variable

[DamienCassou]

Describe the bug

According to:

• feat(vi): allow env to be stubbed to undefined #6359
• https://github.com/vitest-dev/vitest/blob/main/docs/api/vi.md#vistubenv-vi-stubenv

I would expect

vi.stubEnv("foo", undefined);

to delete process.env.foo.

This was working fine until vitest 4: now, passing undefined as value to stubEnv() doesn't seem to do anything anymore.

Reproduction

In this PR of a dedicated repo, you will see that the same test:

• passes in vitest 3
• fails in vitest 4
• fails in vitest 5-beta

The test fails in vitest 4 and 5-beta because the initial value of the environment variable remains after the call to stubEnv(..., undefined)

System Info

System:
    OS: Linux 6.17 Ubuntu 24.04.4 LTS 24.04.4 LTS (Noble Numbat)
    CPU: (4) x64 AMD EPYC 7763 64-Core Processor
    Memory: 14.62 GB / 15.61 GB
    Container: Yes
    Shell: 5.2.21 - /bin/bash
  Binaries:
    Node: 24.16.0 - /opt/hostedtoolcache/node/24.16.0/x64/bin/node
    Yarn: 1.22.22 - /usr/local/bin/yarn
    npm: 11.13.0 - /opt/hostedtoolcache/node/24.16.0/x64/bin/npm
  Browsers:
    Chrome: 148.0.7778.178
    Chromium: 148.0.7778.0
    Firefox: 151.0.1
    Firefox Developer Edition: 151.0.1
  npmPackages:
    vitest: ^4.1.8 => 4.1.8

Used Package Manager

npm

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