MINOR: Removed the RSM and RLMM classpath config validator (#14358)

- RSM and RLMM classpath can be empty since it's optional so removed the non-empty string validator
- Fix getting the `localTieredStorage` by brokerId after stopping a broker.

Reviewers: Christo Lolov <lolovc@amazon.com>, Luke Chen <showuon@gmail.com>, Satish Duggana <satishd@apache.org>

storage/src/main/java/org/apache/kafka/server/log/remote/storage/RemoteLogManagerConfig.java

@@ -171,7 +171,7 @@ public final class RemoteLogManagerConfig {
                                   REMOTE_STORAGE_MANAGER_CLASS_NAME_DOC)
                   .define(REMOTE_STORAGE_MANAGER_CLASS_PATH_PROP, STRING,
                                   null,
-                                  new ConfigDef.NonEmptyString(),
+                                  null,
                                   MEDIUM,
                                   REMOTE_STORAGE_MANAGER_CLASS_PATH_DOC)
                   .define(REMOTE_LOG_METADATA_MANAGER_CLASS_NAME_PROP,
@@ -183,7 +183,7 @@ public final class RemoteLogManagerConfig {
                   .define(REMOTE_LOG_METADATA_MANAGER_CLASS_PATH_PROP,
                                   STRING,
                                   null,
-                                  new ConfigDef.NonEmptyString(),
+                                  null,
                                   MEDIUM,
                                   REMOTE_LOG_METADATA_MANAGER_CLASS_PATH_DOC)
                   .define(REMOTE_LOG_METADATA_MANAGER_LISTENER_NAME_PROP, STRING,

storage/src/test/java/org/apache/kafka/server/log/remote/storage/LocalTieredStorage.java

@@ -561,4 +561,8 @@ public final class LocalTieredStorage implements RemoteStorageManager {
         }
         return SEGMENT;
     }
+
+    public int brokerId() {
+        return brokerId;
+    }
 }

storage/src/test/java/org/apache/kafka/tiered/storage/TieredStorageTestContext.java

@@ -271,11 +271,18 @@ public final class TieredStorageTestContext implements AutoCloseable {
 
     public LocalTieredStorageSnapshot takeTieredStorageSnapshot() {
         int aliveBrokerId = harness.aliveBrokers().head().config().brokerId();
-        return LocalTieredStorageSnapshot.takeSnapshot(remoteStorageManagers.get(aliveBrokerId));
+        return LocalTieredStorageSnapshot.takeSnapshot(remoteStorageManager(aliveBrokerId));
     }
 
     public LocalTieredStorageHistory tieredStorageHistory(int brokerId) {
-        return remoteStorageManagers.get(brokerId).getHistory();
+        return remoteStorageManager(brokerId).getHistory();
+    }
+
+    public LocalTieredStorage remoteStorageManager(int brokerId) {
+        return remoteStorageManagers.stream()
+                .filter(rsm -> rsm.brokerId() == brokerId)
+                .findFirst()
+                .orElseThrow(() -> new IllegalArgumentException("No remote storage manager found for broker " + brokerId));
     }
 
     public List<LocalTieredStorage> remoteStorageManagers() {