Conditions in yaml pipeline like "eq" and "ne" is not working

I have created a pipeline build variable "svctag" and assigned its value to blank (empty string). Now trying to execute below script inside a job in yaml file. Every time it prints "svctag is not blank". Not sure what I am doing wrong. Please help.

- $:
   - script: echo 'svctag is blank'
- $:
   - script: echo 'svctag is not blank'