Enable null to remove server_message

[jimmyhong94]

Description

An empty server message in config.yaml would cause a NullPointerException when logging in (after selecting your character to play) since the server message would be null while the code expects a string.

This change prevents an empty server_message in config.yaml causing the exception.

Error found when trying an empty server_message:

java.lang.NullPointerException: Cannot invoke "String.getBytes(java.nio.charset.Charset)" because "value" is null
        at net.packet.ByteBufOutPacket.writeString(ByteBufOutPacket.java:74)
        at tools.PacketCreator.serverMessage(PacketCreator.java:1281)
        at tools.PacketCreator.serverMessage(PacketCreator.java:1220)
        at net.server.channel.Channel.addPlayer(Channel.java:271)
        at net.server.channel.handlers.PlayerLoggedinHandler.handlePacket(PlayerLoggedinHandler.java:228)
        at client.Client.channelRead(Client.java:221)
        at io.netty.channel.AbstractChannelHandlerContext.fireChannelRead(AbstractChannelHandlerContext.java:357)
        at ...

Error is now removed and server message on top of the view is gone when the config has a null server message.

Checklist before requesting a review

• I have performed a self-review of my code
• I have tested my changes
• I have added unit tests that prove my changes work

Screenshots

Server message removed when null / does not run into NullPointerException:

Server message showing still with my changes (intended):