Fixed Deprecated function issue

src/Controller/JsonapiTaxonomyTree.php

@@ -55,9 +55,10 @@ class JsonapiTaxonomyTree extends ControllerBase {
     $request = \Drupal::request();
     $queryString = $request->getQueryString();
     $queryParams = [];
-    if (!empty($queryString)) {
+    if ($queryString !== null && $queryString !== '') {
       parse_str($queryString, $queryParams);
-    }
+  }
+  
 
     $this_resource_type = 'taxonomy_term--' . $vid;
     if (isset($queryParams['fields'][$this_resource_type])) {
@@ -71,9 +72,10 @@ class JsonapiTaxonomyTree extends ControllerBase {
     $queryString = http_build_query($queryParams);
     $baseUrl = \Drupal::service('url_generator')
       ->generateFromRoute('jsonapi.taxonomy_term--' . $vid . '.collection', [], ['absolute' => TRUE]);
-    if (!empty($queryString)) {
-      $baseUrl = $baseUrl . '?' . $queryString;
+      if ($queryString !== null && $queryString !== '') {
+        $baseUrl = $baseUrl . '?' . $queryString;
     }
+    
     $jsonapi_data = [];
     $nextUrl = $baseUrl;