ParentageResolved flag needs to be updated for workflows with no parents too
**Impact of the bug**
ReqMgr2

**Describe the bug**
As reported in this JIRA:
https://its.cern.ch/jira/browse/CMSCOMPPR-16710

there are workflows that are not marked as parentage resolved, thus blocking Unified from announcing workflows.

**How to reproduce it**
Run StepChain workflow producing only one dataset from scratch, and move it to closed-out.

**Expected behavior**
The ReqMgr2 cherrypy thread should go through all the StepChain workflows and mark them with:
```
ParentageResolved = true
```
when there is no parentage information to be updated, besides the normal operation of the thread.

**Additional context and error message**
Example of workflow producing - from scratch - only a MINIAODSIM dataset, thus no parentage information needs to be updated:
https://cmsweb.cern.ch/reqmgr2/fetch?rid=cmsunified_task_EXO-RunIIFall18wmLHEGS-02833__v1_T_200220_091412_4231
diff --git a/src/python/WMCore/ReqMgr/CherryPyThreads/StepChainParentageFixTask.py b/src/python/WMCore/ReqMgr/CherryPyThreads/StepChainParentageFixTask.py
index 681d2844a..0b3e74885 100644
--- a/src/python/WMCore/ReqMgr/CherryPyThreads/StepChainParentageFixTask.py
+++ b/src/python/WMCore/ReqMgr/CherryPyThreads/StepChainParentageFixTask.py
@@ -21,9 +21,8 @@ def getChildDatasetsForStepChainMissingParent(reqmgrDB, status):
     for reqName, info in results.items():
 
         for dsInfo in info.values():
-            if dsInfo["ParentDset"]:
-                for childDS in dsInfo["ChildDsets"]:
-                    requestsByChildDataset[childDS].add(reqName)
+            for childDS in dsInfo["ChildDsets"]:
+                requestsByChildDataset[childDS].add(reqName)
     return requestsByChildDataset
 
 
['src/python/WMCore/ReqMgr/CherryPyThreads/StepChainParentageFixTask.py']