[HttpClient] fix "undefined variable"

Author: nicolas-grekas

Response/CurlResponse.php

@@ -180,7 +180,7 @@ public function __destruct()
         } catch (HttpExceptionInterface $e) {
             throw $e;
         } finally {
-            if (null !== $e) {
+            if ($e ?? false) {
                 throw $e;
             }
 

Response/NativeResponse.php

@@ -91,7 +91,7 @@ public function __destruct()
         } catch (HttpExceptionInterface $e) {
             throw $e;
         } finally {
-            if (null !== $e) {
+            if ($e ?? false) {
                 throw $e;
             }