more changes to better handle undef vars

This is only needed if we make undefined vars an error as proposed in
issue #4163 but it's a good idea anyway.

share/functions/pushd.fish

@@ -18,10 +18,13 @@ function pushd --description 'Push directory to stack'
         end
     end
 
+    set -q dirstack
+    or set -g dirstack
+
     # emulate bash: an empty pushd should switch the top of dirs
-    if test (count $argv) -eq 0
+    if not set -q argv[1]
         # check that the stack isn't empty
-        if test (count $dirstack) -eq 0
+        if not set -q dirstack[1]
             echo "pushd: no other directory"
             return 1
         end