Trouble writing fish if statement

So I have the following if statement (originally from my .zshrc):

if [ -n "${NVIM_LISTEN_ADDRESS+x}" ]; then
  alias h='nvr -o'
  alias v='nvr -O'
  alias t='nvr --remote-tab'
fi

Now I am trying to port this across to my config.fish, I understand that it is to do with the [ on the if but I can't quite figure out the syntax.