Fix tls_key_file check

[Praying]

!tls_cert_file.is_empty() in src/bin/server.rs is duplicated，remove it.

[CLAassistant]

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[yongman]

@Praying @sunxiaoguang the tls_ca_cert_file can be empty if tls_auth_client is not enabled, so tls_ca_cert_file should not be checked here.