Fix undefined variable error (matrix_synapse_id_servers_public)

This only gets triggered if:
- the Synapse role is used standalone and the default values are used
- the whole playbook is used, with `matrix_mxisd_enabled: false`

roles/matrix-synapse/defaults/main.yml

@@ -64,6 +64,8 @@ matrix_synapse_registration_shared_secret: "{{ matrix_synapse_macaroon_secret_ke
 matrix_synapse_allow_guest_access: false
 matrix_synapse_form_secret: "{{ matrix_synapse_macaroon_secret_key }}"
 
+matrix_synapse_id_servers_public: ['matrix.org', 'vector.im']
+
 # The list of identity servers to use for Synapse.
 # We assume this role runs standalone without a local Identity server, so we point Synapse to public ones.
 # This most likely gets overwritten later, so that a local Identity server is used.