Treat undefined env var as strict mode

An undefined SYMFONY_DEPRECATION_HELPER environment variable translates
to false, and that was previously interpreted as 0, which means strict
mode.
This restores backwards compatibility with the previous behavior, which
got broken in 1c73f9cfed .

src/Symfony/Bridge/PhpUnit/DeprecationErrorHandler.php

@@ -241,6 +241,10 @@ class DeprecationErrorHandler
             return $this->configuration = Configuration::fromNumber($mode);
         }
 
+        if (!$mode) {
+            return $this->configuration = Configuration::fromNumber(0);
+        }
+
         return $this->configuration = Configuration::fromUrlEncodedString((string) $mode);
     }