Action fails when there are no SAM changes

**Describe the bug**
When a change is pushed to main which doesn't update the SAM template, the deployment job will fail.

**To Reproduce**
Steps to reproduce the behavior:
The previous workflow must have completed successfully, and the repo hasn't been updated.
1. Go to Actions
2. Click on workflow "SAM Validate, Build, Test, Deploy"
3. Choose "Re-run jobs"

This job will fail

**Expected behavior**
The running job should succeed

**Screenshots**
Step "SAM deploy" fails with the following error:
(note that I ran this from my own fork, so the stack name is different to what you will expect)

A possible fix: Change the last command in template.yml to the following:

sam deploy --template-file .aws-sam/build/template.yaml \
--stack-name ${{ steps.vars.outputs.environment }} \
--s3-bucket ${{ steps.vars.outputs.environment }} \
--parameter-overrides \
'ParameterKey=Name,ParameterValue=example \
ParameterKey=Version,ParameterValue=${{ steps.vars.outputs.version }}' \
--capabilities CAPABILITY_IAM CAPABILITY_NAMED_IAM \
|| echo "Nothing to deploy, no changes"

Start with template.yml and the “SAM Validate, Build, Test, Deploy” workflow, focusing on the failing “SAM deploy” step. Reproduce the failure by rerunning a completed workflow without SAM template changes, then verify that the job succeeds and reports no deployment changes when there is nothing to deploy.

Written by the indexing model from the issue text.