tls: Provide default object for _tlsOptions when options are undefined

Prevents this error when `options` is not set.

    _tls_wrap.js:205
      var credentials = options.credentials || crypto.createCredentials();
                               ^
    TypeError: Cannot read property 'credentials' of undefined
        at TLSSocket._init (_tls_wrap.js:205:28)
        at new TLSSocket (_tls_wrap.js:186:10)