Check if property exists Groovy

I am using multi-branch plugin in Jenkins and polling my git repo to see if there are any changes. My problem is that when it polls, my $GIT_BRANCH_NAME variable doesn't exist and the build fails.

groovy.lang.MissingPropertyException: No such property: GIT_BRANCH_NAME for class: groovy.lang.Binding
    at groovy.lang.Binding.getVariable(Binding.java:63)

I tried checking for

if (!bindings.hasProperty(GIT_BRANCH_NAME)){
echo "Why is this not working"
env.GIT_BRANCH_NAME = sh(script: "git rev-parse HEAD", returnStdout: true).trim()
}

and

 if (!project.hasProperty(GIT_BRANCH_NAME)){
    echo "Why is this not working"
    env.GIT_BRANCH_NAME = sh(script: "git rev-parse HEAD", returnStdout: true).trim()
    }

It doesn't work though. Any help is appreciated

Solution

Here you go:

if (!bindings.properties."${GIT_BRANCH_NAME}")){
    echo "Why is this not working"
    env.GIT_BRANCH_NAME = sh(script: "git rev-parse HEAD", returnStdout: true).trim()
}

UPD:

As far as I understood the problem is that your class doesn't have neither bindings, nor project and GIT_BRANCH_NAME variables declared (or they're not passed into the binding from outside). Is it possible to publish the entire class and probably the script executor(binding or like that)?