i have create pull request https://github.com/jenkinsci/amazon-ecs-plugin/pull/31

How to reproduce this problem:

Make sure that amazon ecs plugin is the only one agent that active (by setting master executor number to 0, disable other cloud, ..., etc)
Create a template (just use jenkinsci/jnlp-slave image) with arbitary label (or without label)
Create simple job (without label) and run it
BOOM!!, ecs plugin will keep trying to create ecs task and the job will not scheduled to any those agent because the agent is exclusive

right now, the only way to prevent this from happening is to make sure that every job have label