Detect if a log field exists

[AThebrand]

Hi everyone. 

Recently I'm writing some detection rules in YARA-L for my company. 

I have a rule that simply match the following: 

$selection.target.process.file.full_path = /\/bin\/bash/ and not

            (

                $selection.target.ip="127.0.0.1" or

                $selection.target.ip="0.0.0.0"

            )

This rule generates a lot of false positives every time the field target.ip is missing. I would like to trigger the rule only when that field exists and it's not equal certain values. 

So.. how can I check the existence of a field in YARA-L?

Thanks