Parameter null value is not allowed

Prerequisites

• Write a descriptive title.
• Make sure you are able to repro it on the latest version
• Search the existing issues.

Summary

While developing the coalesce function and writing the documentation, I stumbled on the following error from example 3:

Parser: Parameter 'customValue' not found in context.

It looks like parameter default values set to null are not allowed.

Steps to reproduce

Run the following example from the branch comparison-function-coalesce:

$in = @'
$schema: https://aka.ms/dsc/schemas/v3/bundled/config/document.json
parameters:
  customValue:
    type: string
    defaultValue: null
  timeout:
    type: int
    defaultValue: 0
resources:
- name: Configuration with fallbacks
  type: Microsoft.DSC.Debug/Echo
  properties:
    output:
      configValue: "[coalesce(parameters('customValue'), 'default-config')]"
      timeout:     "[coalesce(parameters('timeout'), 30)]"
'@
dsc config get --input $in

Expected behavior

The example would have returned null on `configValue`

Actual behavior

The error: Parser: Parameter 'customValue' not found in context occurs

Error details

Environment data

Name                           Value
----                           -----
PSVersion                      7.5.2
PSEdition                      Core
GitCommitId                    7.5.2
OS                             Microsoft Windows 10.0.26100
Platform                       Win32NT
PSCompatibleVersions           {1.0, 2.0, 3.0, 4.0…}
PSRemotingProtocolVersion      2.3
SerializationVersion           1.1.0.1
WSManStackVersion              3.0

Version

dsc 3.2.0-preview.3

Visuals

No response

Start by running the dsc config get --input reproduction from the comparison-function-coalesce branch and trace parameter lookup for customValue when its defaultValue is null. Compare the result with the expected null configValue, then add or update coverage for null parameter defaults and confirm the existing timeout fallback still works.