Mill export fails if there is only test scope files

[yadavan88]

Version(s)
1.4.0

Describe the bug
Exporting a scala-cli project to Mill with only test files generates an invalid mill project.

To Reproduce

//> using dep org.scalameta::munit::1.0.0
class MyTests extends munit.FunSuite {
  test("foo") {
    assert(2 + 2 == 4)
    println("hello")
  }
}

Export this. Gets invalid mill project

Expected behaviour
A valid and working mill project after export