Pipelines unable to parse service if one of the service variables is empty.

Summary

Pipelines unable to parse service if one of the service variables is empty.

Steps to Reproduce

1. Create a parallel step
2. Add service under the steps
3. Creat service variable and let the variable empty

Expected Results

Pipelines should return a more generic error that the variable is empty

Actual Results

Pipeline return a general error without much information