Schemes can be generated containing a `TestableReference` with no children

If you generate a scheme with the `TestAction` node as follows:

```xml






```

then Xcode will crash on launch. This is, without a doubt, a bug in Xcode but we should generate "valid" XML anyway.

This is generated when running `pod install` with our Podfile. I can't share the Podfile unfortunately, but it may be useful to know that the broken scheme belongs to a demo app which is referenced as an `appspec` (not something I'm particularly familiar with).

This is 100% reproducible in both generation and Xcode crashing.

Start by tracing scheme generation invoked by pod install for an appspec and inspect how the TestAction emits TestableReference nodes. Compare the generated XML with the supplied example; done means generated schemes no longer contain a TestableReference without children.

Written by the indexing model from the issue text.