How to allow an empty string for a runtime parameter?

Question

I'm just starting to work with runtime parameters in Azure Pipelines and there's something I can't quite figure out. Considering this Azure Pipelines YAML:

parameters:
- name: deployEnvironment
  displayName: Select your target environment.
  type: string
  default: Build_only
  values:
  - Build_only
  - TST
  - PP
  - P
- name: releaseName
  type: string
  default: ''

steps:
- task: ....

Why is releaseName a required parameter? I was hoping that by specifying default: '' it would be optional to be left empty. The documentation doesn't mention if parameters can be made optional.

Following up on Kryzstof's answer, I experimented a little further and it seems that a string consisting only of whitespaces is interpreted as empty:

It seems that this single whitespace is interpreted as empty (I've also tried multiple whitespaces).

parameters:
- name: myString
  type: string
  default: ' '

steps:
- task: PowerShell@2
  inputs:
    targetType: inline
    script: |
      $MS = $ENV:MS
      Write-Host "myString value is '$MS'"
      Write-Host "Its length is $($MS.Length)"
      Write-Host "is it null or empty? $([System.String]::IsNullOrEmpty($MS))"
      Write-Host "Is it null or whitespace? $([System.String]::IsNullOrWhiteSpace($MS))"
  env:
    MS: ${{ parameters.myString }}

This yields:

myString value is '' Its length is 0 is it null or empty? True Is it null or whitespace? True

Answer 1

This is really strange. But if you put a space ' ' instead of an empty string '', then you will be able to trigger pipeline, even deleting that space from field.