Add comment for the matrix_client_element_logout_redirect_url variable

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/defaults/main.yml

@@ -319,4 +319,6 @@ matrix_client_element_location_sharing_map_style_content_sources_localsource_til
 # Leave blank, if map does not require attribution.
 matrix_client_element_location_sharing_map_style_content_sources_localsource_attribution: ""
 
+# Optional URL to redirect the user to after they have logged out.
+# See https://github.com/element-hq/element-web/blob/develop/docs/config.md#sso-setup
 matrix_client_element_logout_redirect_url: ''

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 }},