Coverage is not being reported when there are no test files

[odockal]

Describe the bug

I would expect that code coverage is calculated/reported even though there are not test files present. Then it would mean that coverage is zero. Instead, No test files found, exiting with code 0 warning is printed out.

Reproduction

Checkout https://github.com/odockal/zero-coverage-project
Run npm run build
Run npm run test
ASSERT: no code coverage is provided
Rename src/test.ts to src/test.spec.ts
Run npm run test
ASSERT: Now the coverage is provided (zero).

System Info

System:
    OS: Linux 6.1 Fedora Linux 37 (Workstation Edition)
    CPU: (12) x64 Intel(R) Core(TM) i7-10850H CPU @ 2.70GHz
    Memory: 20.42 GB / 31.11 GB
    Container: Yes
    Shell: 5.2.15 - /bin/bash
  Binaries:
    Node: 16.16.0 - ~/.nvm/versions/node/v16.16.0/bin/node
    Yarn: 1.22.19 - ~/.yarn/bin/yarn
    npm: 8.11.0 - ~/.nvm/versions/node/v16.16.0/bin/npm
  Browsers:
    Chrome: 110.0.5481.177
    Firefox: 110.0
  npmPackages:
    @vitest/coverage-c8: ^0.29.5 => 0.29.5 
    vitest: ^0.29.5 => 0.29.5

Used Package Manager

npm

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[AriPerkkio]

This request makes sense - especially when passWithNoTests: true is used. Other JS test runners seem to behave similarly. Let's add support for this.