Don't crash when saving an attachment whose name is only periods.

Sources/Testing/Attachments/Attachment.swift

@@ -545,7 +545,7 @@ extension Attachment where AttachableValue: ~Copyable {
     } catch {
       // Split the extension(s) off the preferred name. The first component in
       // the resulting array is our base name.
-      var preferredNameComponents = preferredName.split(separator: ".")
+      var preferredNameComponents = preferredName.split(separator: ".", omittingEmptySubsequences: false)
       let firstPreferredNameComponent = preferredNameComponents[0]
 
       while true {

Tests/TestingTests/AttachmentTests.swift

@@ -164,6 +164,9 @@ struct AttachmentTests {
     String(repeating: "a", count: maximumNameCount),
     String(repeating: "a", count: maximumNameCount + 1),
     String(repeating: "a", count: maximumNameCount + 2),
+    ".",
+    "..",
+    "...",
   ]) func writeAttachmentWithBadName(name: String) throws {
     let attachableValue = MySendableAttachable(string: "<!doctype html>")
     let attachment = Attachment(attachableValue, named: name)
@@ -176,6 +179,29 @@ struct AttachmentTests {
     try compare(attachableValue, toContentsOfFileAtPath: filePath)
   }
 
+  @Test func writeAttachmentWithEmptyPreferredName() throws {
+    struct MyAttachableWithEmptyName: Attachable, Sendable {
+      var string: String
+
+      func withUnsafeBytes<R>(for attachment: borrowing Attachment<Self>, _ body: (UnsafeRawBufferPointer) throws -> R) throws -> R {
+        try Array(string.utf8).withUnsafeBytes(body)
+      }
+
+      func preferredName(for attachment: borrowing Attachment<Self>, basedOn suggestedName: String) -> String {
+        ""
+      }
+    }
+    let string = "<!doctype html>"
+    let attachment = Attachment(MyAttachableWithEmptyName(string: string))
+
+    let filePath = try attachment.write(toFileInDirectoryAtPath: temporaryDirectory())
+    defer {
+      remove(filePath)
+    }
+    let file = try Testing.FileHandle(forReadingAtPath: filePath)
+    #expect(try file.readToEnd() == Array(string.utf8))
+  }
+
   @Test func fileSystemPathIsSetAfterWritingViaEventHandler() async throws {
     let attachableValue = MySendableAttachable(string: "<!doctype html>")
     try await confirmation("Attachment detected") { valueAttached in