Fix "missing argument" error when calling tab without args

plugins/tab/tab.fish

@@ -12,13 +12,15 @@ function tab -d 'Open the current directory (or any other directory) in a new ta
   set -l cmd ''
   set -l cdto $PWD
 
-  if test (count $argv) -gt 0 -a -d $argv[1]
+  if test (count $argv) -gt 0
+    if test -d $argv[1]
       pushd . >/dev/null
       cd $argv[1]
       set cdto $PWD
       set -e argv[1]
       popd >/dev/null
     end
+  end
 
   if test (count $argv) -gt 0
     set cmd "; $argv"