Fix the issue where the lack of a default value for ssl_options in listeners results in startup failure. For example, such command(EMQX_LISTENERS__WSS__DEFAULT__BIND='0.0.0.0:8089' ./bin/emqx console) would have caused a crash before.
EMQX_LISTENERS__WSS__DEFAULT__BIND='0.0.0.0:8089' ./bin/emqx console