Fix rare NPE in skin handling code (#3357)

core/src/main/java/org/geysermc/geyser/skin/SkinManager.java

@@ -258,8 +258,15 @@ public class SkinManager {
             JsonNode skinObject = GeyserImpl.JSON_MAPPER.readTree(new String(Base64.getDecoder().decode(encodedJson), StandardCharsets.UTF_8));
             JsonNode textures = skinObject.get("textures");
 
-            if (textures != null) {
+            if (textures == null) {
+                return null;
+            }
+
             JsonNode skinTexture = textures.get("SKIN");
+            if (skinTexture == null) {
+                return null;
+            }
+
             String skinUrl = skinTexture.get("url").asText().replace("http://", "https://");
 
             boolean isAlex = skinTexture.has("metadata");
@@ -272,8 +279,6 @@ public class SkinManager {
 
             return new GameProfileData(skinUrl, capeUrl, isAlex);
         }
-            return null;
-        }
 
         /**
          * @return default skin with default cape when texture data is invalid, or the Bedrock player's skin if this