Improve empty pipeline/release parameter support

[dmitryserbin]

• Add V3 empty pipeline parameter value validation - empty value is NOT allowed due to ADO limitations
• Add V2 empty release variable value support - empty value is allowed
• Updated docs
• Add tests

V2

- task: releaseorchestrator@2
  displayName: Release Orchestrator
  inputs:
    ...
    releaseVariables: |
     My-Variable-One=My-Value-One
     My-Variable-Two=

V3

Supplying My-Variable-Two= in V3 should throw an error.

See #86 for details