Simplify auto_join_mxid_localpart population

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

@@ -1541,9 +1541,7 @@ autocreate_auto_join_rooms: {{ matrix_synapse_autocreate_auto_join_rooms|to_json
 # Note that, if the room already exists, this user must be joined and
 # have the appropriate permissions to invite new members.
 #
-{% if matrix_synapse_auto_join_mxid_localpart %}
-auto_join_mxid_localpart: {{ matrix_synapse_auto_join_mxid_localpart }}
-{% endif %}
+auto_join_mxid_localpart: {{ matrix_synapse_auto_join_mxid_localpart | to_json }}
 
 # When auto_join_rooms is specified, setting this flag to false prevents
 # guest accounts from being automatically joined to the rooms.