Empty string variable will not be subtitude during Run customised option process

Currently, if you have a empty string value for a plan variable in the Run customised process the plan value is not updated. Steps to replicate the problem:

1. Create a plan variable with value foo
2. Run a job with the plan variable
3. After the job build process ended visit the job again click on "Run customised"
4. Now replace the plan value with empty string

The value variable will be substitute with "foo: instead of empty string