Use Nullable annotation from JSpecify instead of JetBrains.

Replaced the `@Nullable` annotation from JetBrains with the one provided by JSpecify. This aligns with the project's shift towards JSpecify annotations for nullability handling. No functional changes were introduced.

src/main/java/net/thenextlvl/character/plugin/model/EmptyLootTable.java

@@ -5,8 +5,8 @@
 import org.bukkit.inventory.ItemStack;
 import org.bukkit.loot.LootContext;
 import org.bukkit.loot.LootTable;
-import org.jetbrains.annotations.Nullable;
 import org.jspecify.annotations.NullMarked;
+import org.jspecify.annotations.Nullable;
 
 import java.util.Collection;
 import java.util.List;