changelog(prepare-artifact): treat empty existing-filename as unset (#3314)

CLI parsers (Argh, since #3202) forward `--existing-changelog-filename ""`
as the empty string instead of null, so the existing `!= null` guard let
empty filenames through. `Path.GetFileName("")` returns `""`, then
`Path.Combine(OutputDir, "")` collapses to `OutputDir` itself, and the
artifact YAML write target becomes the directory path.

On Linux this fails with EACCES:

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

Switch the guard to `!string.IsNullOrEmpty` and add a regression test.
A complementary fix in elastic/docs-actions stops forwarding the flag
when the value is empty.

Repro: elastic/cloud PR #154858 / run 25721575364

Co-authored-by: Cursor <cursoragent@cursor.com>

Author: cotti

src/services/Elastic.Changelog/Evaluation/ChangelogPrepareArtifactService.cs

@@ -47,11 +47,17 @@ public async Task<bool> PrepareArtifact(IDiagnosticsCollector collector, Prepare
 
 			if (sourceYaml != null)
 			{
-				changelogFilename = input.ExistingChangelogFilename != null
-					? _fileSystem.Path.GetFileName(input.ExistingChangelogFilename)
+				// Treat empty as unset: CLI parsers (e.g. Argh) forward `--flag ""`
+				// as the empty string instead of null. An empty filename here would
+				// make Path.Combine(OutputDir, "") collapse to OutputDir itself and
+				// turn the artifact write into a write against the directory path,
+				// which fails with EACCES on Linux.
+				var hasExistingFilename = !string.IsNullOrEmpty(input.ExistingChangelogFilename);
+				changelogFilename = hasExistingFilename
+					? _fileSystem.Path.GetFileName(input.ExistingChangelogFilename!)
 					: _fileSystem.Path.GetFileName(sourceYaml);
 
-				if (input.ExistingChangelogFilename != null)
+				if (hasExistingFilename)
 					_logger.LogInformation("Reusing existing filename {Filename} for stable path on branch", changelogFilename);
 
 				var destYaml = _fileSystem.Path.Combine(input.OutputDir, changelogFilename);

tests/Elastic.Changelog.Tests/Evaluation/ChangelogPrepareArtifactServiceTests.cs

@@ -232,6 +232,26 @@ public async Task PrepareArtifact_ExistingFilename_RenamesStagingFileToMatch()
 		metadata.ChangelogFilename.Should().Be("1735689600-old-title.yaml");
 	}
 
+	[Fact]
+	public async Task PrepareArtifact_EmptyExistingFilename_FallsBackToStagingFilename()
+	{
+		// Regression: CLI parsers (Argh) forward `--existing-changelog-filename ""`
+		// as the empty string instead of null. An empty filename used to make
+		// Path.Combine(OutputDir, "") collapse to OutputDir itself and write the
+		// artifact YAML at the directory path → EACCES on Linux.
+		await SetupStagingYaml("1735700000-new-title.yaml");
+		await SetupConfig();
+		var service = CreateService();
+		var args = DefaultArgs() with { ExistingChangelogFilename = string.Empty };
+
+		var result = await service.PrepareArtifact(Collector, args, CancellationToken.None);
+
+		result.Should().BeTrue();
+		FileSystem.File.Exists(Path.Join(OutputDir, "1735700000-new-title.yaml")).Should().BeTrue();
+		var metadata = ReadMetadata();
+		metadata.ChangelogFilename.Should().Be("1735700000-new-title.yaml");
+	}
+
 	[Fact]
 	public async Task PrepareArtifact_MissingStagingYaml_StatusError()
 	{