Testing the latest version of the pipeline plugins and it appears that it's no longer possible to set empty environment variables.  The following works in some current declarative scripts I have still using v1.0.2:

pipeline{
    environment{
       EMAIL_RECIPIENTS         = 'me@me.com' 
       EMAIL_RECIPIENTS_SUCCESS = ''
    }
    
    (some stages and steps)

    post {
        success {
            echo 'Send Success message...'
            buildEmailer (currentBuild.result, EMAIL_RECIPIENTS, EMAIL_RECIPIENTS_SUCCESS)
        }
        failure {
            echo 'Send Failed message...'
            buildEmailer (currentBuild.result, EMAIL_RECIPIENTS, EMAIL_RECIPIENTS_SUCCESS) 
        } 
    }
}

I have emailing logic in shared library (buildEmailer) and do some processing if the incoming variable is blank. The above causes no trouble in plugin version 1.0.2, but in 1.1.2, I get:

groovy.lang.MissingPropertyException: No such property: EMAIL_RECIPIENTS_SUCCESS for class: WorkflowScript
at org.codehaus.groovy.runtime.ScriptBytecodeAdapter.unwrap(ScriptBytecodeAdapter.java:53)
at org.codehaus.groovy.runtime.ScriptBytecodeAdapter.getProperty(ScriptBytecodeAdapter.java:458)
at com.cloudbees.groovy.cps.sandbox.DefaultInvoker.getProperty(DefaultInvoker.java:33)
at com.cloudbees.groovy.cps.impl.PropertyAccessBlock.rawGet(PropertyAccessBlock.java:20)
at WorkflowScript.run(WorkflowScript:127)
...

However, the script is OK if I move things around and set the variables inside a script {} section inside a stage{}. The reason this particular value is set but blank, is that I let this value be optional and let developers update the desired property to what they want in the build job. I could process some default value in the environment section, but since the scripts works with variables in a script section, and it was working in 1.0.2, it seems like this could be a bug with the environment section.