[Bugfix] Prevent error translator returning the translation key

Fixes a change in behaviour introduced in Laravel 7.28.

Author: lindyhopchris

CHANGELOG.md

@@ -2,6 +2,12 @@
 All notable changes to this project will be documented in this file. This project adheres to
 [Semantic Versioning](http://semver.org/) and [this changelog format](http://keepachangelog.com/).
 
+## Unreleased
+
+### Fixed
+- The error translator will now detect if the translated value is identical to the translation
+key path, and return `null` when it is. This fixes behaviour that changed in Laravel 7.28.
+
 ## [2.1.0] - 2020-09-04
 
 ### Added

src/Document/Error/Translator.php

@@ -529,11 +529,13 @@ public function call(\Closure $closure, ...$args): ErrorInterface
      */
     protected function trans(string $key, string $member, array $replace = [], ?string $locale = null)
     {
-        return $this->translator->get(
-            "jsonapi::errors.{$key}.{$member}",
+        $value = $this->translator->get(
+            $key = "jsonapi::errors.{$key}.{$member}",
             $replace,
             $locale
         ) ?: null;
+
+        return ($key !== $value) ? $value : null;
     }
 
     /**