Enhance configuration data validation and add new utility functions

- Added validation for empty values in Assert-ConfigurationDataParameter.
- Introduced Assert-ConfigurationDataSecretReference for secret reference validation.
- Implemented Test-ConfigurationDataExpressionParentheses to validate expression syntax.
- Added Test-ConfigurationDataMissingReferenceError to check for missing parameter or variable definitions.
- Updated Invoke-ConfigurationDataExpression to handle new validation logic.
- Revised Readme.md to reflect changes in secret reference handling.

Private/Assert-ConfigurationDataParameter.ps1

@@ -42,6 +42,10 @@ function Assert-ConfigurationDataParameter {
         return
     }
 
+    if((Test-ConfigurationDataValueIsEmpty -Value $Value) -and (-not $Required)){
+        return
+    }
+
     Assert-ConfigurationDataParameterType -Name $Name -Definition $Definition -Value $Value
     Assert-ConfigurationDataParameterAllowedValue -Name $Name -Definition $Definition -Value $Value