Simplify logout_redirect_url templating and make it safer

Using `to_json` to make it safer.

Judging by the code here 0b24d33c64/src/Lifecycle.ts (L1026-L1032)
it doesn't seem like it leaving `logout_redirect_url` empty will hurt, so this patch removes the `if` block.

roles/custom/matrix-client-element/templates/config.json.j2

@@ -40,9 +40,7 @@
 	{% if matrix_client_element_location_sharing_enabled %}
 	"map_style_url": {{ matrix_client_element_map_style_url | to_json }},
 	{% endif %}
-	{% if matrix_client_element_logout_redirect_url != '' %}
-	"logout_redirect_url": "{{ matrix_client_element_logout_redirect_url }}",
-	{% endif %}
+	"logout_redirect_url": {{ matrix_client_element_logout_redirect_url | to_json }},
 	"branding": {
 		"auth_footer_links": {{ matrix_client_element_branding_auth_footer_links | to_json }},
 		"auth_header_logo_url": {{ matrix_client_element_branding_auth_header_logo_url | to_json }},