fix NPE in SpawnGroup.write (#8584)

* Update SpawnGroup.java

* Update contributors

* No Objects.isNull

core/assets/contributors

@@ -154,4 +154,5 @@ zenonet
 AyuKo-o
 JojoFR1
 Xasmedy
-xStaBUx
+xStaBUx
+WayZer

core/src/mindustry/game/SpawnGroup.java

@@ -138,7 +138,7 @@ public class SpawnGroup implements JsonSerializable, Cloneable{
         shieldScaling = data.getFloat("shieldScaling", 0);
         unitAmount = data.getInt("amount", 1);
         spawn = data.getInt("spawn", -1);
-        if(data.has("payloads")) payloads = Seq.with(json.readValue(String[].class, data.get("payloads"))).map(s -> content.unit(s));
+        if(data.has("payloads")) payloads = Seq.with(json.readValue(String[].class, data.get("payloads"))).map(content::unit).removeAll(t -> t == null);
         if(data.has("items")) items = json.readValue(ItemStack.class, data.get("items"));