lib/utilities: handle unbound parameter

Expressly handle unbound $BASH_IT_GREP when testing for value

lib/utilities.bash

@@ -58,7 +58,7 @@ _bash-it-array-dedup() {
 
 # Outputs a full path of the grep found on the filesystem
 _bash-it-grep() {
-  if [[ -z "${BASH_IT_GREP}" ]] ; then
+  if [[ -z "${BASH_IT_GREP:-}" ]] ; then
     export BASH_IT_GREP="$(which egrep || which grep || '/usr/bin/grep')"
   fi
   printf "%s " "${BASH_IT_GREP}"