NullPointerException in RepositoryDelegatorFunction

### Description of the bug:

I just got the following error:
```
(09:47:10) FATAL: bazel crashed due to an internal error. Printing stack trace:
java.lang.RuntimeException: Unrecoverable error while evaluating node 'REPOSITORY_DIRECTORY:@@_main~portage~alchemist' (requested by nodes 'PACKAGE_LOOKUP:@@_main~portage~alchemist//')
at com.google.devtools.build.skyframe.AbstractParallelEvaluator$Evaluate.run(AbstractParallelEvaluator.java:550)
at com.google.devtools.build.lib.concurrent.AbstractQueueVisitor$WrappedRunnable.run(AbstractQueueVisitor.java:414)
at java.base/java.util.concurrent.ForkJoinTask$AdaptedRunnableAction.exec(Unknown Source)
at java.base/java.util.concurrent.ForkJoinTask.doExec(Unknown Source)
at java.base/java.util.concurrent.ForkJoinPool$WorkQueue.topLevelExec(Unknown Source)
at java.base/java.util.concurrent.ForkJoinPool.scan(Unknown Source)
at java.base/java.util.concurrent.ForkJoinPool.runWorker(Unknown Source)
at java.base/java.util.concurrent.ForkJoinWorkerThread.run(Unknown Source)
Caused by: java.lang.NullPointerException
at com.google.common.base.Preconditions.checkNotNull(Preconditions.java:903)
at com.google.devtools.build.lib.rules.repository.RepositoryDelegatorFunction.fetchRepository(RepositoryDelegatorFunction.java:439)
at com.google.devtools.build.lib.rules.repository.RepositoryDelegatorFunction.compute(RepositoryDelegatorFunction.java:205)
at com.google.devtools.build.skyframe.AbstractParallelEvaluator$Evaluate.run(AbstractParallelEvaluator.java:461)
... 7 more
09:47:11.673: INFO: run: /mnt/host/source/chromite/bin/bazel run //bazel/portage/tools/process_artifacts -- '--build-events-jsonl=/tmp/chromeos_bazel_build_events.json' '--archive-logs=/tmp/allpackages_action_logs.tar.gz' '--prebuilts=/tmp/prebuilts.bzl'
09:47:11.780: DEBUG: Ensure file: fuchsia/third_party/bazelisk/${os}-${arch} version:2@1.20.0
Starting local Bazel server and connecting to it...
```

See the following [build](https://logs.chromium.org/logs/chromeos/buildbucket/cr-buildbucket/8746588296079805713/+/u/install_packages/call_chromite.api.SysrootService_InstallPackages/call_build_API_script/stdout) (Google internal only).

### Which category does this issue belong to?

Core

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

I have no idea...

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

Linux

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

bazel-7.1.0-linux-x86_64...

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

_No response_

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

_No response_

### Is this a regression? If yes, please try to identify the Bazel commit where the bug was introduced.

_No response_

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

_No response_

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

_No response_

Start with the supplied stack trace and inspect RepositoryDelegatorFunction.fetchRepository at line 439, then review the linked ChromeOS build log for the repository and package context. Establish a minimal reproduction for the Linux Bazel 7.1.0 failure; done means the repository evaluation no longer crashes with a NullPointerException and the behavior is covered by a regression test.

Written by the indexing model from the issue text.