Using Beanshell Assertion to test a JSON value

[RL]

I'm trying to test a JSON value that may be = null.

jp@gc - JSON Path Assertion doesn't appear to return anything or define the
variable if it encounters a null value, so I was trying to get it to work
like this:

- use jp@gc - JSON Path Extractor to try to extract the value to a variable.
- use Beanshell Assertion to try to test to see if the variable exists or
has a null value
- If the variable does not exist, or exists and has a null value, I know
that the JSON value was null.

I haven't written any scripts with Jmeter before, so I may be doing
something obviously wrong, but here is what I've tried:

try {
  String blobvalue = vars.get("myValue");
  log.info("myValue =" + myValue);
}
catch (e) {
  log.info( "caught exception: "+e );
  string myValue = null;
}


if (myValue.length() > 0  && myValue != 0 ){
 Failure = true;
 FailureMessage = "myValue was " + myValue + ".";
}
else{
  Failure = false;
}

For this test, a null value actually passes the test.

The problem I'm running into is that the try/catch block doesn't work.
Instead, I see the following message in the log:

jmeter.assertions.BeanShellAssertion:
org.apache.jorphan.util.JMeterException: Error invoking bsh method: eval
Sourced file: inline evaluation of: ``import
org.apache.jmeter.util.JMeterUtils; try {   //myValue = JMeterUtils.get . .
. '' : Attempt to resolve method: length() on undefined variable or class
name: myValue

Can anyone tell me what I'm doing wrong and/or how to get this test of a
null JSON value to work?

Thanks very much!