JENKINS-59911 Chained assignment is not working in pipeline

Found that groovy in pipeline is not supporting the chained assignment. In tests it's working well, but in pipeline value becomes null.

Get the details here: https://issues.jenkins-ci.org/browse/JENKINS-59911

Author: Sergei Parshev

src/com/griddynamics/devops/mpl/MPLConfig.groovy

@@ -158,8 +158,10 @@ public class MPLConfig implements Map, Serializable {
       else
         throw new MPLException("Invalid config key path '${key_list[i-1] = '<'+key_list[i-1]+'>'; key_list.join('.')}': marked key value type '${parent?.getClass()}' is not suitable to set the nested variable")
 
-      if( value == null )
-        value = parent[key] = [:]
+      if( value == null ) {
+        parent[key] = [:]
+        value = parent[key]
+      }
     }
 
     parent[key] = val