Auto merge of #24342 - petosorus:null_responseXml, r=jdm

XMLHttpRequest.responseXML returns null when a network error occurs

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- [x] `./mach test-tidy` does not report any errors
- [x] These changes fix #24285 (GitHub issue number if applicable)

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components/script/dom/xmlhttprequest.rs

@@ -1292,12 +1292,17 @@ impl XMLHttpRequest {
 
     // https://xhr.spec.whatwg.org/#document-response
     fn document_response(&self) -> Option<DomRoot<Document>> {
-        // Step 1
+        // Caching: if we have existing response xml, redirect it directly
         let response = self.response_xml.get();
         if response.is_some() {
             return self.response_xml.get();
         }
 
+        // Step 1
+        if self.response_status.get().is_err() {
+            return None;
+        }
+
         let mime_type = self.final_mime_type();
         // TODO: prescan the response to determine encoding if final charset is null
         let charset = self.final_charset().unwrap_or(UTF_8);