Fix HashMap default value for step parameters

   https://mirantis.jira.com/browse/PRODX-3627

Change-Id: I5aa13174cac7af0ac521d9f8cb311b853b1aef48
diff --git a/src/com/mirantis/mk/Workflow.groovy b/src/com/mirantis/mk/Workflow.groovy
index d13301c..aa38c0c 100644
--- a/src/com/mirantis/mk/Workflow.groovy
+++ b/src/com/mirantis/mk/Workflow.groovy
@@ -168,12 +168,13 @@
 
             def job_name = step['job']
             def job_parameters = [:]
+            def step_parameters = step['parameters'] ?: [:]
             if (step['inherit_parent_params'] ?: false) {
                 // add parameters from the current job for the child job
                 job_parameters << getJobDefaultParameters(env.JOB_NAME)
             }
             // add parameters from the workflow for the child job
-            job_parameters << step['parameters']
+            job_parameters << step_parameters
 
             // Collect job parameters and run the job
             def job_info = runJob(job_name, job_parameters, global_variables, propagate)