style: add ValidateNotNullOrEmpty to ManifestHelpers mandatory string params (#8)

Completes the cross-repo validator alignment with PowerShellModuleTemplate#23.
PR #7 (2026-05-10) added the validator to `Split-SemVerString -VersionString`;
this commit covers the remaining six mandatory `[string]` parameters across
`Compare-SemVerPrerelease`, `Test-VersionComparison`, and `Test-VersionConstraint`.

Behavior for valid inputs is unchanged. Misuse (null or empty version/prerelease
strings) now fails at parameter binding instead of producing downstream
NullReferenceExceptions inside the comparison logic.

Author: tablackburn

tests/ManifestHelpers.psm1

@@ -89,9 +89,11 @@ function Compare-SemVerPrerelease {
     [OutputType([int])]
     param(
         [Parameter(Mandatory = $true)]
+        [ValidateNotNullOrEmpty()]
         [string]$FirstPrerelease,
 
         [Parameter(Mandatory = $true)]
+        [ValidateNotNullOrEmpty()]
         [string]$SecondPrerelease
     )
 
@@ -205,13 +207,15 @@ function Test-VersionComparison {
     [OutputType([bool])]
     param(
         [Parameter(Mandatory = $true)]
+        [ValidateNotNullOrEmpty()]
         [string]$FirstVersion,
 
         [Parameter(Mandatory = $false)]
         [AllowNull()]
         [string]$FirstPrerelease,
 
         [Parameter(Mandatory = $true)]
+        [ValidateNotNullOrEmpty()]
         [string]$SecondVersion,
 
         [Parameter(Mandatory = $false)]
@@ -322,9 +326,11 @@ function Test-VersionConstraint {
     [OutputType([bool])]
     param(
         [Parameter(Mandatory = $true)]
+        [ValidateNotNullOrEmpty()]
         [string]$ManifestVersion,
 
         [Parameter(Mandatory = $true)]
+        [ValidateNotNullOrEmpty()]
         [string]$RequirementsVersion,
 
         [Parameter(Mandatory = $true)]