Fix: BSD expr syntax error on empty $NNNLVL (#546)

misc/quitcd/quitcd.fish

@@ -4,7 +4,7 @@
 
 function n --description 'support nnn quit and change directory'
     # Block nesting of nnn in subshells
-    if test -n NNNLVL
+    if test -n "$NNNLVL"
         if [ (expr $NNNLVL + 0) -ge 1 ]
             echo "nnn is already running"
             return