Add simple test for non-empty workflow, make it work

src/Microsoft.PowerShell.Workflow.ServiceCore/ServiceCore/WorkflowCore/PSActivityBase.cs

@@ -3956,7 +3956,12 @@ namespace Microsoft.PowerShell.Activities
                     {
                         try
                         {
-                            runspace.SessionStateProxy.PSVariable.Set(name, value);
+                            var psvar = runspace.SessionStateProxy.PSVariable.Get(name);
+                            if (psvar == null || (psvar.Options & ScopedItemOptions.ReadOnly) == 0)
+                            {
+                                // don't try to overwrite read-only variables values
+                                runspace.SessionStateProxy.PSVariable.Set(name, value);
+                            }
                         }
                         catch (PSNotSupportedException)
                         {

test/fullclr/PowerShellGithubDev.Tests.ps1

@@ -30,7 +30,8 @@ Describe 'build.psm1 and powershell.exe' {
         }
 
         It 'loads Microsoft.PowerShell.Workflow.ServiceCore.dll' {
-            workflow wfTest {}; wfTest ## Trigger the loading of ServiceCore.dll
+            workflow wfTest { Split-Path $pwd }
+            wfTest | Should Not Be $null ## Also trigger the loading of ServiceCore.dll
             [Microsoft.PowerShell.Workflow.PSWorkflowJob].Assembly.Location | Should Be (
                 Join-Path $env:DEVPATH Microsoft.PowerShell.Workflow.ServiceCore.dll)
         }