CompositeByteBuf.nioBuffers(...) must not return an empty ByteBuffer arr...

...ay

Motivation:

CompositeByteBuf.nioBuffers(...) returns an empty ByteBuffer array if the specified length is 0. This is not consistent with other ByteBuf implementations which return an ByteBuffer array of size 1 with an empty ByteBuffer included.

Modifications:

Make CompositeByteBuf.nioBuffers(...) consistent with other ByteBuf implementations.

Result:

Consistent and correct behaviour of nioBufffers(...)
Shouldn't this be `Unpooled.EMPTY_BUFFER`?