Fish shell: Check if argument is provided for function

count is the right way to do this. For the common case of checking whether there are any arguments, you can use its exit status:

function fcd
    if count $argv > /dev/null
        open $argv
    else
        open $PWD
    end
end

To answer your second question, test -d $argv returns true if $argv is empty, because POSIX requires that when test is passed one argument, it must “Exit true (0) if $1 is not null; otherwise, exit false”. So when $argv is empty, test -d $argv means test -d which must exit true because -d is not empty! Argh!

edit Added a missing end, thanks to Ismail for noticing