[#2680] ChannelOutboundBuffer.nioBuffers() should always return non-null array as stated in javadocs

Motivation:

At the moment ChannelOutboundBuffer.nioBuffers() returns null if something is contained in the ChannelOutboundBuffer which is not a ByteBuf. This is a problem for two reasons:
 1 - In the javadocs we state that it will never return null
 2 - We may do a not optimal write as there may be things that could be written via gathering writes

Modifications:

Change ChannelOutboundBuffer.nioBuffers() to never return null but have it contain all ByteBuffer that were found before the non ByteBuf. This way we can do a gathering write and also conform to the javadocs.

Result:

Better speed and also correct implementation in terms of the api.