Parse empty sign lines as empty JSON strings.

Since the server doesn't bother validating sign text, and
the client validates it by crashing. Note that this attempts
to wrap strings as JSON, but hasn't been fully tested.

worldedit-core/src/main/java/com/sk89q/worldedit/util/gson/GsonUtil.java

@@ -19,6 +19,7 @@
 
 package com.sk89q.worldedit.util.gson;
 
+import com.google.gson.Gson;
 import com.google.gson.GsonBuilder;
 import com.sk89q.worldedit.Vector;
 
@@ -41,4 +42,8 @@ public final class GsonUtil {
         return gsonBuilder;
     }
 
+    private static final Gson gson = new Gson();
+    public static String stringValue(String s) {
+        return gson.toJson(s);
+    }
 }