function foo {
  PARAM1=$1
  PARAM2="$2"
  VAR=$3
  if[[ -z "$VAR" ]]; then
   # code here
  else
   # other code here
  fi
}

foo "x" "y" "blah"
foo "x" "y" "true"
foo "y" "y" "1"

if [ ! -z "true" ]; then printf 'not emptyn'; fi
not empty

if [ ! -z "false" ]; then printf 'not emptyn'; fi
not empty

if "false"; then printf 'not emptyn'; fi
#                  <-- empty result

if ! "false"; then printf 'not falsen'; fi
not false

if "true"; then printf 'not falsen'; fi
not false