Convert validUnusedVariableNames to array late

Previously the property was converted to an array when the Sniff was
registered. This could cause issues if the property was changed (I know
it's unlikely but it can happen in tests pretty easily) after
registration. Furthermore, it was convered to an array and then used to
replace the original string property, which introduces risk by both
mutating a property during an object lifecycle and even changing its type.

With this change, we never change the property from a string, and
instead we convert it to an array just before it is used.

VariableAnalysis/Sniffs/CodeAnalysis/VariableAnalysisSniff.php

@@ -48,10 +48,6 @@ class VariableAnalysisSniff implements Sniff {
   public $validUnusedVariableNames = null;
 
   public function register() {
-    if (!empty($this->validUnusedVariableNames)) {
-      $this->validUnusedVariableNames =
-        preg_split('/\s+/', trim($this->validUnusedVariableNames));
-    }
     return [
       T_VARIABLE,
       T_DOUBLE_QUOTED_STRING,
@@ -124,7 +120,10 @@ protected function getOrCreateVariableInfo($varName, $currScope) {
     $scopeInfo = $this->getOrCreateScopeInfo($currScope);
     if (!isset($scopeInfo->variables[$varName])) {
       $scopeInfo->variables[$varName] = new VariableInfo($varName);
-      if ($this->validUnusedVariableNames && in_array($varName, $this->validUnusedVariableNames)) {
+      $validUnusedVariableNames = (empty($this->validUnusedVariableNames))
+        ? []
+        : preg_split('/\s+/', trim($this->validUnusedVariableNames));
+      if (in_array($varName, $validUnusedVariableNames)) {
         $scopeInfo->variables[$varName]->ignoreUnused = true;
       }
     }