[Bugfix] Ensure test response content is always a string

Symfony can return a string or false for the response content.
This fix ensures that false is converted to an empty string, as
v4 of the JSON:API testing package type-hints the content as a
string.

Author: lindyhopchris

CHANGELOG.md

@@ -3,6 +3,15 @@
 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 Symfony response class can return `false` for the response content. This caused a fatal error when the content
+  was passed to JSON:API assertions - as the assertion methods type-hint the content as a `string` in version 4 of that
+  dependency. This has been fixed by adding a `TestResponse::getContent()` method that returns an empty string if the
+  Symfony method returns `false`.
+
 ## [1.1.0] - 2022-02-08
 
 ### Added

src/TestResponse.php

@@ -24,7 +24,6 @@
 
 class TestResponse extends BaseTestResponse
 {
-
     use HasHttpAssertions;
 
     /**
@@ -79,6 +78,22 @@ public function getId(): ?string
         return $this->jsonApi('/data/id');
     }
 
+    /**
+     * Get the response content.
+     *
+     * @return string
+     */
+    public function getContent(): string
+    {
+        $content = $this->baseResponse->getContent();
+
+        if (false === $content) {
+            return '';
+        }
+
+        return $content;
+    }
+
     /**
      * @return string|null
      */