Do I need to check HTTP::header exists before checking value?

I feel like this is a simple question, but I can't seem to find a definitive answer...

 

To check the value of a particular HTTP header, I can do this:

 

if { [HTTP::header Origin] ends_with "example.com" } {
    ...do stuff here...
}

My basic question is "What happens is the Origin header is not passed?"...

 

The spec says that HTTP::header will return null if the header called does not exist. What does this mean for my if statement - is null treated the same as false in an if statement?

 

If the above code example will be problematic if Origin is not passed, can I write the code as a single line, like this:

 

if { [HTTP::header exists Origin] and [HTTP::header Origin] ends_with "example.com" } {
    ...do stuff here...
}

or does it need to be two separate nested if statements:

 

if { [HTTP::header exists Origin] } {
    if {[HTTP::header Origin] ends_with "example.com" } {
        ...do stuff here...
    }
}