Prevent creation of empty directories in merge

Author: ffried

Sources/SwiftDocCUtilities/Action/Actions/Merge/MergeAction.swift

@@ -65,6 +65,8 @@ struct MergeAction: AsyncAction {
                 let fromDirectory = archive.appendingPathComponent(directoryToCopy, isDirectory: true)
                 let toDirectory = targetURL.appendingPathComponent(directoryToCopy, isDirectory: true)
 
+                guard fileManager.directoryExists(atPath: fromDirectory.path) else { continue }
+
                 // Ensure that the destination directory exist in case the first archive didn't have that kind of pages.
                 // This is necessary when merging a reference-only archive with a tutorial-only archive.
                 try? fileManager.createDirectory(at: toDirectory, withIntermediateDirectories: false, attributes: nil)

Tests/SwiftDocCUtilitiesTests/MergeActionTests.swift

@@ -996,14 +996,13 @@ class MergeActionTests: XCTestCase {
         Output.doccarchive/
         ├─ data/
         │  ├─ documentation.json
-        │  ├─ documentation/
-        │  │  ├─ first.json
-        │  │  ├─ first/
-        │  │  │  ╰─ article.json
-        │  │  ├─ second.json
-        │  │  ╰─ second/
-        │  │     ╰─ article.json
-        │  ╰─ tutorials/
+        │  ╰─ documentation/
+        │     ├─ first.json
+        │     ├─ first/
+        │     │  ╰─ article.json
+        │     ├─ second.json
+        │     ╰─ second/
+        │        ╰─ article.json
         ├─ downloads/
         │  ├─ First/
         │  ╰─ Second/