Merge pull request #7132 from stelcodes/fix-fish-manpath

Fix fish shell MANPATH creation

scripts/nix-profile.fish.in

@@ -28,7 +28,9 @@ if test -n "$HOME" && test -n "$USER"
     # Only use MANPATH if it is already set. In general `man` will just simply
     # pick up `.nix-profile/share/man` because is it close to `.nix-profile/bin`
     # which is in the $PATH. For more info, run `manpath -d`.
+    if set --query MANPATH
       set --export --prepend --path MANPATH "$NIX_LINK/share/man"
+    end
 
     fish_add_path --prepend --global "$NIX_LINK/bin"
     set --erase NIX_LINK