NPE with mod show_extension

[laurentlb]

Description of the bug:

$ bazel mod show_extension @bazel_skylib//foo%bar.bzl
FATAL: bazel crashed due to an internal error. Printing stack trace:
java.lang.NullPointerException: Cannot invoke "com.google.common.collect.ImmutableSetMultimap.keySet()" because the return value of "com.google.common.collect.ImmutableMap.get(Object)" is null
	at com.google.devtools.build.lib.bazel.bzlmod.modcommand.ModExecutor.displayExtension(ModExecutor.java:472)
	at com.google.devtools.build.lib.bazel.bzlmod.modcommand.ModExecutor.showExtension(ModExecutor.java:145)
	at com.google.devtools.build.lib.bazel.commands.ModCommand.execInternal(ModCommand.java:549)
	at com.google.devtools.build.lib.bazel.commands.ModCommand.exec(ModCommand.java:132)
	at com.google.devtools.build.lib.runtime.BlazeCommandDispatcher.execExclusively(BlazeCommandDispatcher.java:683)
	at com.google.devtools.build.lib.runtime.BlazeCommandDispatcher.exec(BlazeCommandDispatcher.java:252)
	at com.google.devtools.build.lib.server.GrpcServerImpl.executeCommand(GrpcServerImpl.java:607)
	at com.google.devtools.build.lib.server.GrpcServerImpl.lambda$run$1(GrpcServerImpl.java:676)
	at io.grpc.Context$1.run(Context.java:566)
	at java.base/java.util.concurrent.ThreadPoolExecutor.runWorker(Unknown Source)
	at java.base/java.util.concurrent.ThreadPoolExecutor$Worker.run(Unknown Source)
	at java.base/java.lang.Thread.run(Unknown Source)

Note that @bazel_skylib exists, but foo and bar.bzl don't.

This still crashes with last_green.

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?

release 7.1.0

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