Summary
When jenkins runs a parameterized job, if one of the string parameters has a value of 0 and the groovy postbuild task attempts to read that parameter, it will show up as null.

To Reproduce

1. Create a Jenkins Job
2. 'This Build is parameterized'
3. Add one string parameter (eg: name some-param) with default value 0
4. Add a post-build action for groovy postbuild
5. Add the following code:
   ```
   def varName = "some-param";
   def tempVal = manager.build.buildVariables.get(varName )
   if(tempVal == null || tempVal.equals("")){
   manager.listener.logger.println("${varName} not found: '${tempVal}'")
   }
   ```
6. Run the parameterized build, leaving the default value
7. View the console and note the log output of the empty param