[Console][PhpUnitBridge][VarDumper] Fix NO_COLOR empty value handling

Author: alexandre-daubois

Dumper/CliDumper.php

@@ -606,7 +606,7 @@ private function hasColorSupport($stream): bool
         }
 
         // Follow https://no-color.org/
-        if (isset($_SERVER['NO_COLOR']) || false !== getenv('NO_COLOR')) {
+        if ('' !== ($_SERVER['NO_COLOR'] ?? getenv('NO_COLOR') ?: '')) {
             return false;
         }