Don't try to deserialize a null string in legacy method

Spigot-Server-Patches/0010-Adventure.patch

@@ -1612,7 +1612,7 @@ index 3a782294c15e2c2592976858e28ec33f049cb65e..772c6b2fa50deac27e5ed112e6d9f9dd
      public String getCustomName() {
          TileEntityEnchantTable enchant = this.getSnapshot();
 diff --git a/src/main/java/org/bukkit/craftbukkit/block/CraftSign.java b/src/main/java/org/bukkit/craftbukkit/block/CraftSign.java
-index 81f6bf5533288ed90e2f1f4d421d54195d9650c7..4daa39abc7ec1cae91d917a747efb192c2ca05eb 100644
+index 81f6bf5533288ed90e2f1f4d421d54195d9650c7..d742f72215f6a9a2b562182fd57fd9acc59c9818 100644
 --- a/src/main/java/org/bukkit/craftbukkit/block/CraftSign.java
 +++ b/src/main/java/org/bukkit/craftbukkit/block/CraftSign.java
 @@ -13,8 +13,10 @@ import org.bukkit.craftbukkit.util.CraftChatMessage;
@@ -1688,7 +1688,7 @@ index 81f6bf5533288ed90e2f1f4d421d54195d9650c7..4daa39abc7ec1cae91d917a747efb192
      public void setLine(int index, String line) throws IndexOutOfBoundsException {
 -        getLines()[index] = line;
 +        this.loadLines();
-+        this.lines.set(index, io.papermc.paper.adventure.PaperAdventure.LEGACY_SECTION_UXRC.deserialize(line)); // Paper
++        this.lines.set(index, line != null ? io.papermc.paper.adventure.PaperAdventure.LEGACY_SECTION_UXRC.deserialize(line) : net.kyori.adventure.text.Component.empty()); // Paper
      }
  
      @Override