Remove identity server section from .well-known/matrix/client if there is no identity server

Riot used to be fine with it being blank but now it complains. This creates an ugly looking comma when there is an identity server configured but I guess that's fine.

roles/matrix-base/templates/static-files/well-known/matrix-client.j2

@@ -2,8 +2,10 @@
 {
 	"m.homeserver": {
 		"base_url": "{{ matrix_homeserver_url }}"
-	},
+	}
+	{% if matrix_identity_server_url %},
 	"m.identity_server": {
 		"base_url": "{{ matrix_identity_server_url }}"
 	}
+	{% endif %}
 }