Crash on combination of scoped flag, transitions and invalid PROJECT.scl

### Description of the bug:

I have found that the following combination of code causes a Precondition failure here - https://cs.opensource.google/bazel/bazel/+/release-9.0.0rc6:src/main/java/com/google/devtools/build/lib/analysis/producers/BuildConfigurationKeyProducer.java;l=375

PROJECT.scl with an entry for a buildable_unit with isdefault=True, but no project_directories at all

label_flag (or string_flag or whatever) with scope="project"

in-graph transition on that flag

### Which category does this issue belong to?

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### What's the simplest, easiest way to reproduce this bug? Please provide a minimal example if possible.

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### Which operating system are you running Bazel on?

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### What is the output of `bazel info release`?

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### If `bazel info release` returns `development version` or `(@non-git)`, tell us how you built Bazel.

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### What's the output of `git remote get-url origin; git rev-parse HEAD` ?

```text

```

### If this is a regression, please try to identify the Bazel commit where the bug was introduced with bazelisk --bisect.

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### Have you found anything relevant by searching the web?

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### Any other information, logs, or outputs that you want to share?

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