Fix NPE problems

Motivation:

Now Netty has a few problems with null values.

Modifications:

- Check HAProxyProxiedProtocol in HAProxyMessage constructor and throw NPE if it is null.
If HAProxyProxiedProtocol is null we will set AddressFamily as null. So we will get NPE inside `checkAddress(String, AddressFamily)` and it won't be easy to understand why `addrFamily` is null.
- Check File in DiskFileUpload.toString().
If File is null we will get NPE when calling `toString()` method.
- Check Result<String> in MqttDecoder.decodeConnectionPayload(...).
If `!mqttConnectVariableHeader.isWillFlag() || !mqttConnectVariableHeader.hasUserName() || !mqttConnectVariableHeader.hasPassword()` we will get NPE when we will try to create new instance of MqttConnectPayload.
- Check Unsafe before calling `unsafe.getClass()` in PlatformDependent0 static block.
- Removed unnecessary null check in WebSocket08FrameEncoder.encode(...).
Because `msg.content()` can not return null.
- Removed unnecessary null check in DefaultStompFrame(StompCommand) constructor.
Because we have this check in the super class.
- Removed unnecessary null checks in ConcurrentHashMapV8.removeTreeNode(TreeNode<K,V>).
- Removed unnecessary null check in OioDatagramChannel.doReadMessages(List<Object>).
Because `tmpPacket.getSocketAddress()` always returns new SocketAddress instance.
- Removed unnecessary null check in OioServerSocketChannel.doReadMessages(List<Object>).
Because `socket.accept()` always returns new Socket instance.
- Pass Unpooled.buffer(0) instead of null inside CloseWebSocketFrame(boolean, int) constructor.
If we will pass null we will get NPE in super class constructor.
- Added throw new IllegalStateException in GlobalEventExecutor.awaitInactivity(long, TimeUnit) if it would be called before GlobalEventExecutor.execute(Runnable).
Because now we will get NPE. IllegalStateException would be better in this case.
- Fixed null check in OpenSslServerContext.setTicketKeys(byte[]).
Now we throw new NPE if byte[] is not null.

Result:

Added new null checks when it is necessary, removed unnecessary null checks and fixed some NPE problems.
I think this change is incorrect for DatagramChannels that are connected.