style: remove unreachable IsNullOrEmpty guard in Split-SemVerString

The previous commit added [ValidateNotNullOrEmpty()] to $VersionString,
which makes the in-body IsNullOrEmpty check unreachable under normal
parameter binding — both Copilot and CodeRabbit flagged it as dead
code. Removing the redundant guard.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: tablackburn

tests/ManifestHelpers.psm1

@@ -40,10 +40,6 @@ function Split-SemVerString {
         [string]$VersionString
     )
 
-    if ([string]::IsNullOrEmpty($VersionString)) {
-        throw 'VersionString cannot be empty or null'
-    }
-
     # Strip build metadata per SemVer 2.0.0 — it does not affect precedence and is
     # not valid for [System.Version], so it must be removed before further parsing.
     $coreVersion = ($VersionString -split '\+', 2)[0]