ci: Add conditional check before removing pre-staging resources

[ppawlowski]

Description

This pull request add a conditional check if pre-staging helm release for related PR exists befor removal attempt. This is a temporary workaround for a problem when pre-staging environment was not created. Long-term solution should be fixed as part of linked issue.

Related Issue(s)

https://github.com/FlowFuse/flowfuse/issues/4070

Checklist

• [x] I have read the contribution guidelines
• [ ] Suitable unit/system level tests have been added and they pass
• [ ] Documentation has been updated
  • [ ] Upgrade instructions
  • [ ] Configuration details
  • [ ] Concepts
• [ ] Changes flowforge.yml?
  • [ ] Issue/PR raised on FlowFuse/helm to update ConfigMap Template
  • [ ] Issue/PR raised on FlowFuse/CloudProject to update values for Staging/Production

Labels

• [ ] Includes a DB migration? -> add the area:migration label

[hardillb]

@ppawlowski merge when is suitable