Merge branch '5.0' into 5.1

* 5.0:
  Fix abstract method name in PHP doc block
  Various cleanups
  [HttpClient] fix issues in tests
  Fixes sprintf(): Too few arguments in form transformer
  [Console] Fix QuestionHelper::disableStty()
  [Validator] Use Mime component to determine mime type for file validator
  validate subforms in all validation groups
  Update Hungarian translations
  Add meaningful message when Process is not installed (ProcessHelper)
  [PropertyAccess] Fix TypeError parsing again.
  [TwigBridge] fix fallback html-to-txt body converter
  [Security/Http] fix merge
  [ErrorHandler] fix setting $trace to null in FatalError
  [Form] add missing Czech validators translation
  [Validator] add missing Czech translations
  never directly validate Existence (Required/Optional) constraints

Author: nicolas-grekas

Error/FatalError.php

@@ -72,9 +72,11 @@ public function __construct(string $message, int $code, array $error, int $trace
             'line' => $error['line'],
             'trace' => $trace,
         ] as $property => $value) {
-            $refl = new \ReflectionProperty(\Error::class, $property);
-            $refl->setAccessible(true);
-            $refl->setValue($this, $value);
+            if (null !== $value) {
+                $refl = new \ReflectionProperty(\Error::class, $property);
+                $refl->setAccessible(true);
+                $refl->setValue($this, $value);
+            }
         }
     }