[9.0.0] Fix NPE with remote repo contents cache (#27703)

I haven't been able to reproduce this in a test, but this should fix the
following crash observed while running `bazel info`:
```
FATAL: bazel crashed due to an internal error. Printing stack trace:
java.lang.NullPointerException: Cannot invoke "java.util.concurrent.ExecutorService.shutdownNow()" because "this.materializationExecutor" is null
	at com.google.devtools.build.lib.remote.RemoteExternalOverlayFileSystem.afterCommand(RemoteExternalOverlayFileSystem.java:145)
	at com.google.devtools.build.lib.remote.RemoteModule.afterCommand(RemoteModule.java:1034)
	at com.google.devtools.build.lib.runtime.BlazeRuntime.afterCommand(BlazeRuntime.java:787)
	at com.google.devtools.build.lib.runtime.BlazeCommandDispatcher.execExclusively(BlazeCommandDispatcher.java:807)
	at com.google.devtools.build.lib.runtime.BlazeCommandDispatcher.exec(BlazeCommandDispatcher.java:266)
	at com.google.devtools.build.lib.server.GrpcServerImpl.executeCommand(GrpcServerImpl.java:608)
	at com.google.devtools.build.lib.server.GrpcServerImpl.lambda$run$0(GrpcServerImpl.java:679)
	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)
```

Closes #27690.

PiperOrigin-RevId: 833722608
Change-Id: I88c485a01e5967657ec3b5529a47639b743b18e6

Commit
a7d0e91

Co-authored-by: Fabian Meumertzheim <[email protected]>

Author: bazel-io

src/main/java/com/google/devtools/build/lib/remote/RemoteExternalOverlayFileSystem.java

@@ -135,6 +135,11 @@ public void beforeCommand(
   }
 
   public void afterCommand() {
+    if (cache == null) {
+      // Not all commands cause beforeCommand to be called, but afterCommand is called
+      // unconditionally.
+      return;
+    }
     this.cache = null;
     this.inputPrefetcher = null;
     this.reporter = null;