eve: only output ja3 and ja3s if present

eve: only output ja3 and ja3s if present

This will prevent JSON entries like the following that occur
with the dedault configuration (ja3 deactivated and extended
tls ouput activated):

  "tls": {
    "subject": "C=GB, ST=London, L=London, O=Global Security, OU=IT Department, CN=example.com",
    "issuerdn": "C=GB, ST=London, L=London, O=Global Security, OU=IT Department, CN=example.com",
    "serial": "00:9C:FC:DA:1D:A4:70:87:5D",
    "fingerprint": "b8:18:2d:cb:c9:f8:1a:66:75:13:18:31:24:e0:92:35:42:ab:96:89",
    "version": "TLSv1",
    "notbefore": "2020-05-03T11:07:28",
    "notafter": "2021-05-03T11:07:28",
    "ja3": {},
    "ja3s": {}
  }