zk: Check WIKIROOT before adding n alias for zk

Since this function also requires a WIKIROOT to be available in the
environment, we should only define it if that variable is not empty --
just like all the other zk aliases and functions.

writing/zk/config/sh/alias.d/zk.sh

@@ -1,20 +1,20 @@
 #!/usr/bin/env sh
-#
-
-_zk_wiki() {
-    zk --working-dir="$WIKIROOT" "$@"
-}
-
-n() {
-    if [ $# -eq 0 ]; then
-        _zk_wiki edit -i
-    else
-        _zk_wiki "${@}"
-    fi
-}
 
 # We have a local wiki
 if [ -n "${WIKIROOT}" ]; then
+
+    _zk_wiki() {
+        zk --working-dir="$WIKIROOT" "$@"
+    }
+
+    n() {
+        if [ $# -eq 0 ]; then
+            _zk_wiki edit -i
+        else
+            _zk_wiki "${@}"
+        fi
+    }
+
     # open notes with my vim zettelkasten plugin
     # TODO better implementation conditional on zk.nvim & zettelkasten existing
     # nvim +'lua pcall(require "zk.commands"') --headless +qa 2>&1 or similar to check - but slow