Set default delimiter to space

Since `delim` is not empty by default, this also requires to change
logic for determinig if the split will be done--now it's true if any
of --split or --split-field have been provided.

src/libexec/saturnin-dmenu

 numbered=false
 lines=10
 maybe_split=cat
+delim=" "
 field=2-
 
 while true; do case $1 in
     -l|--lines)     lines=$2; shift 2 ;;
-    --split)        delim=$2; shift 2 ;;
-    --split-field)  field=$2; shift 2 ;;
+    --split)        delim=$2; maybe_split=do_split; shift 2 ;;
+    --split-field)  field=$2; maybe_split=do_split; shift 2 ;;
     --help|--usage) usage             ;;
     *)              break             ;;
 esac done
 
-test -n "$delim"        && maybe_split=do_split
-
 nb="$(saturnin conf -p style.color.nbg)"
 nf="$(saturnin conf -p style.color.nfg)"
 sb="$(saturnin conf -p style.color.sbg)"
 }
 
 do_split() {
-    cut -d$delim -f$field
+    cut "-d$delim" -f$field
 }
 
 dmenu -b -i -l $lines $(style) "$@" | $maybe_split