MINOR: Don't print cleaning group metadata log if empty (apache#19080)

The new group coordinator prints the following line at fixed interval
even if no groups were deleted:

```
Generated 0 tombstone records while cleaning up group metadata in 0 milliseconds. (org.apache.kafka.coordinator.group.GroupCoordinatorShard)
```

The time component has some value in its own but it may be better to not
print if when there are not records in order to reduce the spam in the
logs.

Reviewers: Chia-Ping Tsai <[email protected]>

Author: dajac

group-coordinator/src/main/java/org/apache/kafka/coordinator/group/GroupCoordinatorShard.java

@@ -753,8 +753,11 @@ public CoordinatorResult<Void, CoordinatorRecord> cleanupGroupMetadata() {
             }
         });
 
-        log.info("Generated {} tombstone records while cleaning up group metadata in {} milliseconds.",
-            records.size(), time.milliseconds() - startMs);
+        if (!records.isEmpty()) {
+            log.info("Generated {} tombstone records while cleaning up group metadata in {} milliseconds.",
+                records.size(), time.milliseconds() - startMs);
+        }
+
         // Reschedule the next cycle.
         scheduleGroupMetadataExpiration();
         return new CoordinatorResult<>(records, false);