Do not generate targets for Pods without sources

If a PodTarget should not be build:
- Do not install a target
- Skip the non-existing native targets in Installer
- Do not add linker flags to XCConfig

Close #2916
I'd add a separate context for that, where I would stub the `source_files` in a dedicated `before do … end` block and add further tests, that the vendored frameworks and libraries are still added to the respective build settings.