Apply suggestions from code review

Co-authored-by: Slavi Pantaleev <slavi@devture.com>

roles/custom/matrix-dendrite/templates/dendrite/dendrite.yaml.j2

@@ -193,9 +193,9 @@ client_api:
   recaptcha_private_key: {{ matrix_dendrite_recaptcha_private_key|to_json }}
   recaptcha_bypass_secret: ""
   recaptcha_siteverify_api: {{ matrix_dendrite_recaptcha_siteverify_api|to_json }}
-  recaptcha_api_js_url: ""
-  recaptcha_form_field: ""
-  recaptcha_sitekey_class: ""
+  recaptcha_api_js_url: {{ matrix_dendrite_recaptcha_api_js_url | to_json }}
+  recaptcha_form_field: {{ matrix_dendrite_recaptcha_form_field | to_json }}
+  recaptcha_sitekey_class: {{ matrix_dendrite_recaptcha_sitekey_class | to_json }}
 
   # TURN server information that this homeserver should send to clients.
   turn: