Make OIDC providers if check safer

roles/custom/matrix-synapse/templates/synapse/homeserver.yaml.j2

@@ -2090,7 +2090,7 @@ saml2_config:
 # use 'oidc' for the idp_id to ensure that existing users continue to be
 # recognised.)
 #
-{% if matrix_synapse_oidc_enabled %}
+{% if matrix_synapse_oidc_enabled and matrix_synapse_oidc_providers | length > 0 %}
   # Generic example
   #matrix_synapse_oidc_providers:
   #- idp_id: my_idp