Unexpected "null" in CHAT_JSON message crashes the plugin

[thebirk]

When attempting to parse a CHAT_JSON message where any of the element has a null value the plugin crashes.

[   ScriptRuntime] [16:52:19] [BetterChat]  $FFFCHAT_JSON:{"login":null,"nickname":null,"text":"You do not have access to this command"}
[   ScriptRuntime] [16:52:19] [BetterChat]  Script exception: Json value is not a string!
[   ScriptRuntime] [16:52:19] [BetterChat]    src/ChatLine.as (line 83, column 5)
[   ScriptRuntime] [16:52:19] [BetterChat]      #0  void ChatLine::ParseLine(const string&in line) (src/ChatLine.as line 83)
[   ScriptRuntime] [16:52:19] [BetterChat]      #1  ChatLine::ChatLine(uint id, int64 time, const string&in line) (src/ChatLine.as line 30)
[   ScriptRuntime] [16:52:19] [BetterChat]      #2  ChatLine@ ChatLine(uint id, int64 time, const string&in line) (src/ChatLine.as line 0)
[   ScriptRuntime] [16:52:19] [BetterChat]      #3  void ChatWindow::AddLine(const string&in line) (src/ChatWindow.as line 108)
[   ScriptRuntime] [16:52:19] [BetterChat]      #4  void ChatWindow::OnChatMessage(const string&in line) (src/ChatWindow.as line 171)
[   ScriptRuntime] [16:52:19] [BetterChat]      #5  void ChatMessageLoop() (src/ChatMessageLoop.as line 104)