Remove confusing error when no docs are produced (#149)

Removes a confusing error that started being emitted when `docc convert`
failed to actually produce documentation for an otherwise valid DocC catalog.

This regression was introduced by the enablement of transform for static
hosting by default since that command relies on a `data` subdirectory
existing in the produced documentation archive.

Resolves rdar://91790147.

Author: ethan-kusters

Sources/SwiftDocCUtilities/Action/Actions/Convert/ConvertAction.swift

@@ -377,12 +377,19 @@ public struct ConvertAction: Action, RecreatingContext {
         }
 
         // Process Static Hosting as needed.
-        if transformForStaticHosting, let templateDirectory = htmlTemplateDirectory {
+        if transformForStaticHosting,
+           let templateDirectory = htmlTemplateDirectory,
+           // If this conversion didn't actually produce documentation, then we expect
+           // the creation of this data provider to fail because there will be no 'data' subdirectory
+           // in the documentation output. (r91790147)
+           let dataProvider = try? LocalFileSystemDataProvider(
+               rootURL: temporaryFolder.appendingPathComponent(NodeURLGenerator.Path.dataFolderName)
+           )
+        {
             if indexHTMLData == nil {
                 indexHTMLData = try StaticHostableTransformer.transformHTMLTemplate(htmlTemplate: templateDirectory, hostingBasePath: hostingBasePath)
             }
 
-            let dataProvider = try LocalFileSystemDataProvider(rootURL: temporaryFolder.appendingPathComponent(NodeURLGenerator.Path.dataFolderName))
             let transformer = StaticHostableTransformer(dataProvider: dataProvider, fileManager: fileManager, outputURL: temporaryFolder, indexHTMLData: indexHTMLData!)
             try transformer.transform()
         }

Tests/SwiftDocCUtilitiesTests/ConvertActionTests.swift

@@ -2300,6 +2300,35 @@ class ConvertActionTests: XCTestCase {
 
         expectedOutput.assertExist(at: result.outputs[0], fileManager: testDataProvider)
     }
+    
+    // Tests that the default behavior of `docc convert` on the command-line does not throw an error
+    // when processing a DocC catalog that does not actually produce documentation. (r91790147)
+    func testConvertDocCCatalogThatProducesNoDocumentationDoesNotThrowError() throws {
+        let emptyCatalog = Folder(
+            name: "unit-test.docc",
+            content: [InfoPlist(displayName: "TestBundle", identifier: "com.test.example")]
+        )
+        
+        let temporaryDirectory = try createTemporaryDirectory()
+        let outputDirectory = temporaryDirectory.appendingPathComponent("output", isDirectory: true)
+        let doccCatalogDirectory = try emptyCatalog.write(inside: temporaryDirectory)
+        let htmlTemplateDirectory = try Folder.emptyHTMLTemplateDirectory.write(inside: temporaryDirectory)
+        
+        setenv(TemplateOption.environmentVariableKey, htmlTemplateDirectory.path, 1)
+        defer {
+            unsetenv(TemplateOption.environmentVariableKey)
+        }
+        
+        let convertCommand = try Docc.Convert.parse(
+            [
+                doccCatalogDirectory.path,
+                "--output-path", outputDirectory.path,
+            ]
+        )
+        
+        var action = try ConvertAction(fromConvertCommand: convertCommand)
+        _ = try action.perform(logHandle: .none)
+    }
 
     func testConvertWithCustomTemplates() throws {
         let info = InfoPlist(displayName: "TestConvertWithCustomTemplates", identifier: "com.test.example")