the tls binding throws NPE if there are no options defined

[vordimous]

Describe the bug
Zilla throws a NullPointerException on startup if a tls binding is defined with a proper exit but no options. The workaround was to add a mutual: none param:

  south_tls_client:
    type: tls
    kind: client
    options:
      mutual: none
    exit: south_tcp_client

To Reproduce
try to use a tls binding with no options defined:

  south_tls_client:
    type: tls
    kind: client
    exit: south_tcp_client

[vordimous]

fixed by #611