Check for variable non-existence in conditional task executes task even if variable exists

Issue Summary

This is reproducible on Data Center: (yes) / (no)

Steps to Reproduce

1. Create a conditional task using the condition variable bamboo.XXX not exists
2. Create a variable XXX (either in plan or in global variables)

Expected Results

Task should not be executed

Actual Results

Task is executed

...

Workaround

Currently there is no known workaround for this behavior. A workaround will be added here when available