Correctly validation API calls to mark a backup as completed

Also block modifying a backup that is already marked as completed via the endpoint

app/Http/Requests/Api/Remote/ReportBackupCompleteRequest.php

@@ -12,9 +12,9 @@ class ReportBackupCompleteRequest extends FormRequest
     public function rules()
     {
         return [
-            'successful' => 'boolean',
+            'successful' => 'present|boolean',
             'checksum' => 'nullable|string|required_if:successful,true',
-            'checksum_type' => 'string|required_if:successful,true',
+            'checksum_type' => 'nullable|string|required_if:successful,true',
             'size' => 'nullable|numeric|required_if:successful,true',
         ];
     }