code: private boolean isParallelOn(Plugin plugin) throws MojoExecutionException {
          String value = extractParamValue(plugin, PARALLEL_PARAM_NAME);
          // If value is set and it is not an empty string.
          return value != null && !value.equals("");
      }
docs: Returns true if "parallel" parameter is ON, false
  otherwise. This parameter is ON if the value is different from
  NULL.
  
  https://maven.apache.org/surefire/maven-surefire-plugin/examples/fork-options-and-parallel-execution.html