changelog/submit: skip empty --existing-changelog-filename

[cotti]

Summary

When a PR has no previously-committed changelog file, the evaluate step output existing-changelog-filename is empty, and we were unconditionally forwarding --existing-changelog-filename "" to docs-builder changelog prepare-artifact.

docs-builder's ChangelogPrepareArtifactService treats "" as a real filename:

changelogFilename = input.ExistingChangelogFilename != null
    ? _fileSystem.Path.GetFileName(input.ExistingChangelogFilename)  // GetFileName("") == ""
    : _fileSystem.Path.GetFileName(sourceYaml);

var destYaml = _fileSystem.Path.Combine(input.OutputDir, changelogFilename);
// Path.Combine(".artifacts/changelog-artifact", "") == ".artifacts/changelog-artifact"
await _fileSystem.File.WriteAllTextAsync(destYaml, ...);

The write target ends up being the output directory path itself, and on Linux opening a directory for write returns EACCES:

Unhandled ChangelogPrepareArtifactService exception:
Access to the path '/home/runner/work/cloud/cloud/.artifacts/changelog-artifact' is denied.
System.IO.IOException: Permission denied
  at System.IO.File.OpenHandle(...)
  at System.IO.File.<WriteToFileAsync>...

This change only forwards --existing-changelog-filename when the value is non-empty. A complementary fix in elastic/docs-builder will treat "" as null inside the service so neither side relies on the other.

The regression became visible after docs-builder commit 0d6115b8 migrated the CLI from ConsoleAppFramework to Nullean.Argh, which no longer maps --flag "" to null.

Repro: elastic/cloud PR #154858 / run 25721575364.

Test plan

• Re-run the failing job in elastic/cloud against this branch (or wait for the next changelog-bearing PR) and confirm prepare-artifact succeeds.
• Sanity-check that supplying a real existing-changelog-filename (PR with a prior committed changelog) still works.

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