locator-directory should not return empty string

[cgay]

I think it should be valid to do ensure-directory-exists(relative-path). While fixing dylan-lang/testworks#157 I wrote this test:

// https://github.com/dylan-lang/testworks/issues/157
define test test-test-temp-directory/absolute-path ()
  let dylan = os/environment-variable("DYLAN");
  block ()
    // This value must be a relative path.
    // This test is obviously not thread safe.
    os/environment-variable("DYLAN") := "dylan/root";
    let tmp = test-temp-directory();
    let d1 = subdirectory-locator(tmp, "d1");
    let f1 = merge-locators(as(<file-locator>, "f1"), tmp);
    let f2 = write-test-file(f1, content: "x");
    assert-equal(resolve-locator(f1), resolve-locator(f2));
  cleanup
    os/environment-variable("DYLAN") := dylan;
  end;
end test;

test-temp-directory makes a relative path because I've set DYLAN that way, and then calls ensure-directories-exist with it, which ends up with the following error because locator-directory returns the empty string.

$ $DW/testworks/_build/bin/testworks-test-suite-app --test test-test-temp-directory/absolute-path --debug crashes
Running test test-test-temp-directory/absolute-path:test-directory = dylan/root/_test/cgay-20230115-181036/test-test-temp-directory_absolute-path/
creating directory {<posix-directory-locator> ""}  // I added this debug output to ensure-directories-exist
No such file or directory: Can't create the directory
Backtrace:
  0x7fc7014acd98
  default-handler:dylan:dylan##1 + 0x12
  0x79643a6e616c7964
  default-last-handler:common-dylan-internals:common-dylan##0 + 0x2f6
  error:dylan:dylan##0 + 0x27
  unix-file-error:system-internals:system + 0x227
  %create-directory:system-internals:system + 0x94
  KdoitF193I + 0x9e
  KdoitF193I + 0x71
  KdoitF193I + 0x71
  KdoitF193I + 0x71
  KdoitF193I + 0x71
  ensure-directories-exist:file-system:system##0 + 0x71
  test-temp-directory:testworks:testworks + 0x1b4
  %%test-test-temp-directory/absolute-path:testworks-test-suite:testworks-test-suite + 0x34
  Kanonymous_of_execute_componentF71I + 0x360
  do-with-profiling:common-dylan-internals:common-dylan + 0x2f
  execute-component:%testworks:testworks##1 + 0x244
  maybe-execute-component:%testworks:testworks##0 + 0x113
  run-tests:testworks:testworks + 0x88
  run-or-list-tests:%testworks:testworks + 0xaa
  run-test-application:testworks:testworks + 0x413
  main + 0x19