MINOR: Code cleanup in server module (#22026)

Fix no-op String.join and Javadoc

Reviewers: Mickael Maison <mickael.maison@gmail.com>

Author: nileshkumar3

server/src/main/java/org/apache/kafka/server/FetchSession.java

@@ -90,7 +90,7 @@ public class FetchSession {
 
     static String partitionsToLogString(Collection<TopicIdPartition> partitions, boolean traceEnabled) {
         return traceEnabled
-            ? "(" + String.join(", ", partitions.toString()) + ")"
+            ? partitions.toString()
             : partitions.size() + " partition(s)";
     }
 

server/src/main/java/org/apache/kafka/server/FetchSessionCacheShard.java

@@ -118,7 +118,7 @@ Meter evictionsMeter() {
      * Get a session by session ID.
      *
      * @param sessionId  The session ID.
-     * @return           The session, or None if no such session was found.
+     * @return           The session, or an empty Optional if no such session was found.
      */
     synchronized Optional<FetchSession> get(int sessionId) {
         return Optional.ofNullable(sessions.get(sessionId));
@@ -243,7 +243,7 @@ synchronized Optional<FetchSession> remove(int sessionId) {
      *
      * @param session  The session.
      *
-     * @return         The removed session, or None if there was no such session.
+     * @return         The removed session, or an empty Optional if there was no such session.
      */
     synchronized Optional<FetchSession> remove(FetchSession session) {
         EvictableKey evictableKey;