Forbid mutating empty_json_array

It's definitely still possible (rawset), but should prevent people
accidentally trying to modify it when they shouldn't.

src/main/resources/assets/computercraft/lua/rom/apis/textutils.lua

@@ -240,7 +240,11 @@ local function serializeImpl( t, tTracking, sIndent )
     end
 end
 
-empty_json_array = {}
+empty_json_array = setmetatable({}, {
+    __newindex = function()
+        error("attempt to mutate textutils.empty_json_array", 2)
+    end
+})
 
 local function serializeJSONImpl( t, tTracking, bNBTStyle )
     local sType = type(t)