output/tls: don't pass NULL fingerprint to json logging

src/output-json-tls.c

@@ -139,9 +139,11 @@ static void JsonTlsLogSessionResumed(json_t *js, SSLState *ssl_state)
 
 static void JsonTlsLogFingerprint(json_t *js, SSLState *ssl_state)
 {
+    if (ssl_state->server_connp.cert0_fingerprint) {
         json_object_set_new(js, "fingerprint",
                 json_string(ssl_state->server_connp.cert0_fingerprint));
     }
+}
 
 static void JsonTlsLogSni(json_t *js, SSLState *ssl_state)
 {