NPE with bazel mod show_repo '<root>'

[laurentlb]

Description of the bug:

In any Bazel workspace (even an empty one), run:

$ USE_BAZEL_VERSION=last_green bazel mod show_repo '<root>'
 checking cached actions
    Fetching module extension xcode_configure_extension in @@bazel_tools//tools/osx:xcode_configure.bzl; starting
FATAL: bazel crashed due to an internal error. Printing stack trace:
java.lang.RuntimeException: Unrecoverable error while evaluating node 'BZLMOD_REPO_RULE:@@' (requested by nodes )
    at com.google.devtools.build.skyframe.AbstractParallelEvaluator$Evaluate.run(AbstractParallelEvaluator.java:557)
    at com.google.devtools.build.lib.concurrent.AbstractQueueVisitor$WrappedRunnable.run(AbstractQueueVisitor.java:426)
    at java.base/java.util.concurrent.ForkJoinTask$RunnableExecuteAction.exec(Unknown Source)
    at java.base/java.util.concurrent.ForkJoinTask.doExec(Unknown Source)
    at java.base/java.util.concurrent.ForkJoinPool$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 java.base/java.util.Objects.requireNonNull(Unknown Source)
    at java.base/java.util.Optional.of(Unknown Source)
    at com.google.devtools.build.lib.skyframe.BzlmodRepoRuleFunction.checkRepoFromBazelModules(BzlmodRepoRuleFunction.java:159)
    at com.google.devtools.build.lib.skyframe.BzlmodRepoRuleFunction.compute(BzlmodRepoRuleFunction.java:113)
    at com.google.devtools.build.skyframe.AbstractParallelEvaluator$Evaluate.run(AbstractParallelEvaluator.java:468)
    ... 7 more

Which category does this issue belong to?

External Dependency

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

No response

Which operating system are you running Bazel on?

No response

What is the output of bazel info release?

7.1.0 (or last_green)

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

[Wyverald]

Understandable, since the main repo isn't really a "repo"... we shouldn't crash either way.

This should be a simple fix -- PRs would be welcome :)