Use `computeIfAbsent` instead of an `if`

Signed-off-by: Andrew DeMaria <lostonamountain@gmail.com>

airsonic-main/src/main/java/org/airsonic/player/service/JukeboxJavaService.java

@@ -67,11 +67,7 @@ public class JukeboxJavaService {
                 if (StringUtils.isBlank(mixer)) {
                     mixer = DEFAULT_MIXER_ENTRY_KEY;
                 }
-                List<com.github.biconou.AudioPlayer.api.Player> playersForMixer = activeAudioPlayersPerMixer.get(mixer);
+                List<com.github.biconou.AudioPlayer.api.Player> playersForMixer = activeAudioPlayersPerMixer.computeIfAbsent(mixer, k -> new ArrayList<>());
-                if (playersForMixer == null) {
-                    playersForMixer = new ArrayList<>();
-                    activeAudioPlayersPerMixer.put(mixer, playersForMixer);
-                }
                 playersForMixer.add(newPlayer);
                 foundPlayer = newPlayer;
             }

airsonic-main/src/main/java/org/airsonic/player/service/MusicIndexService.java

@@ -98,11 +98,7 @@ public class MusicIndexService {
 
         for (T artist : artists) {
             MusicIndex index = getIndex(artist, indexes);
-            List<T> artistSet = result.get(index);
+            List<T> artistSet = result.computeIfAbsent(index, k -> new ArrayList<T>());
-            if (artistSet == null) {
-                artistSet = new ArrayList<T>();
-                result.put(index, artistSet);
-            }
             artistSet.add(artist);
         }