[Bro] Error SSL::Info server_name - not present

[Riccardo Bortolameotti]

Hello, 

I am a student, and I started few days ago to approach the bro's
scripting language. 

I am writing a script that needs to get some information regarding SSL
communications. In my script I do:
- local server_n = c$ssl$server_name;
- local subj = c$ssl$subject;
Of course these fields are not always in the table (e.g. in SSLv3
server_name does not exist, therefore no values), then my script raise a
warning while it runs, saying that those values do not exist. 

What can I do to avoid that warning output?

I have tried to do some checks like:

if (c$ssl$server_name=="-" || c$ssl$server_name=="empty")
	do something ...

But it does not work, I do not know how it is represented it is not in
the table.

thank you in advance,

glad to join this community,

R.