Add check for empty ID, to avoid unhandled error where no ID specified

dotask.sh

#!/bin/bash

TMPFILE='/tmp/rofi-dmenu-taskwarrior'

task rofi rc.verbose=nothing > $TMPFILE

TASK=`rofi -dmenu -p "Task: " -i -input $TMPFILE`

TASK_ID=`echo $TASK | cut -f1 -d' '`

echo $TASK_ID > /tmp/dotask-id

[[ ! -z "$var" ]] && task start $TASK_ID