Do not generated Swift files with no code (#988)

Author: crazytonyli

native/swift/Tools/Sources/GenerateCancellable/main.swift

@@ -36,6 +36,9 @@ struct GenerateCancellable: ParsableCommand {
 
         let sourceCode = try String(contentsOf: inputURL, encoding: .utf8)
         let extensionsCode = try generateExtensions(sourceCode: sourceCode, logger: logger)
+        guard !extensionsCode.isEmpty else {
+            return
+        }
 
         try extensionsCode.write(to: outputURL, atomically: true, encoding: .utf8)
 
@@ -216,13 +219,18 @@ class ExtensionGenerator {
             )
         )
 
+        let content = sourceFile.description.trimmingCharacters(in: .whitespacesAndNewlines)
+        guard !content.isEmpty else {
+            return ""
+        }
+
         return """
         // Do not modify. This file is automatically generated.
         // swiftlint:disable all
 
         import Foundation
 
-        \(sourceFile.description)
+        \(content)
         """
     }