The default configuration had the `cacert` option for the
`ssl.external` listener commented out. If one is to set `verify =
verify_peer` in that listener without noticing that `cacert` is
unspecified, all connections will fail with:
```
2022-07-11T11:48:28.540807-03:00 [error] supervisor: 'esockd_connection_sup - <0.3971.0>', errorContext: connection_shutdown, reason: {ssl_error,{options,{cacertfile,[]}}}, offender: [{pid,<0.8225.13>},{name,connection},{mfargs,{emqx_connection,start_link,[[{deflate_options,[]},{max_conn_rate,10000},{active_n,100},{zone,external},{proxy_address_header,<<>>},{proxy_port_header,<<>>},{supported_subprotocols,[]}]]}}]
```
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