Fix missing dstPath in Copy Bundle Resources phases

[Ckitakishi]

Summary

XcodeGen omitted dstPath when generating the "Copy Bundle Resources" copy files phase. Xcode-written projects carry the key with an empty value when no subpath is needed (Embed Frameworks, Embed App Extensions), and Xcode 27.2 rejects a missing value when converting a project to the new project.xcproj format.

The generated phase now uses dstPath: "", consistent with the other copy files phase generation paths.

Reproduction

Requires Xcode 27.2, which added the xcproj format. Xcode 27.0 rejects it with Could not find requested format.

project.yml:

name: Demo

targets:
  App:
    type: application
    platform: iOS
    dependencies:
      - bundle: MyBundle.bundle

xcodegen generate
xcodebuild -project Demo.xcodeproj -convert-project xcproj

Before this change the conversion fails:

xcodebuild: error: Unable to save project: A copy build phase is missing a required value for destination path

Encountered while saving:
• A project named “Demo”
• A native target named “App”
• A copy files build phase named “Copy Bundle Resources”

After it the conversion succeeds and replaces project.pbxproj with project.xcproj. A failing conversion leaves the project untouched.

Testing

• Bundle copy phase tests now assert dstPath == ""
• Updated the TestProject fixture with dstPath = "";
• Ran swift test