Make `user_agent` field optional part of the Sentinel message

User agent header is optional part of HTTP message. Currently user_agent field is sent to server every time even there was no user agent header or the header had an empty value.

To distinguish these there are a few options:

1. don't send user_agent field when there is no header, send empty user_agent value when header has an empty value

2. send msg_pack nil value in user_agent field value, send empty user_agent value when header has an empty value

3. don't send user_agent field when there is no header or the header value is empty

Seninel_meeting_w48

@mprudek @vmyslivec @kkoci