Fix instances where Signs have null lines. Addresses BUKKIT-684

src/main/java/net/minecraft/server/NetServerHandler.java

@@ -1302,6 +1302,9 @@ public class NetServerHandler extends NetHandler {
                 if (!event.isCancelled()) {
                     for (int l = 0; l < 4; ++l) {
                         tileentitysign1.lines[l] = event.getLine(l);
+                        if(tileentitysign1.lines[l] == null) {
+                            tileentitysign1.lines[l] = "";
+                        }
                     }
                     tileentitysign1.isEditable = false;
                 }

src/main/java/org/bukkit/craftbukkit/block/CraftSign.java

@@ -35,7 +35,13 @@ public class CraftSign extends CraftBlockState implements Sign {
         boolean result = super.update(force);
 
         if (result) {
-            System.arraycopy(lines, 0, sign.lines, 0, lines.length);
+            for(int i = 0; i < 4; i++) {
+                if(lines[i] != null) {
+                    sign.lines[i] = lines[i];
+                } else {
+                    sign.lines[i] = "";
+                }
+            }
             sign.update();
         }