Add check that JSON request body exists when required

[MAILPOET-4135]

mailpoet/lib/Automation/API/Request.php

@@ -36,7 +36,13 @@ class Request {
   }
 
   public function getBody(): array {
-    return $this->wpRequest->get_json_params() ?? [];
+    $json = $this->wpRequest->get_json_params();
+
+    /* @phpstan-ignore-next-line hotfix for missing 'null' in WP annotation */
+    if ($json === null) {
+      throw Exceptions::apiNoJsonBody();
+    }
+    return $json;
   }
 
   public function getRawBody(): string {

mailpoet/lib/Automation/Exceptions.php

@@ -3,11 +3,20 @@
 namespace MailPoet\Automation;
 
 use MailPoet\Automation\Exceptions\InvalidStateException;
+use MailPoet\Automation\Exceptions\UnexpectedValueException;
 
 class Exceptions {
+  private const API_NO_JSON_BODY = 'mailpoet_automation_api_no_json_body';
+
   public function __construct() {
     throw new InvalidStateException(
       "This is a static factory class. Use it via 'Exception::someError()' factories."
     );
   }
+
+  public static function apiNoJsonBody(): UnexpectedValueException {
+    return UnexpectedValueException::create()
+      ->withErrorCode(self::API_NO_JSON_BODY)
+      ->withMessage('No JSON body passed.');
+  }
 }