BUG #16020: restore defaults for repeatable vector properties

Repeatable vector properties were not being restored when
vtkSMProperty::RestoreToXMLDefaults() was called because no defaults
for such properties are typically defined in the proxy XML. This patch
handles this case by clearing out the vector elements, setting the
size to zero.

ParaViewCore/ServerManager/Core/vtkSMVectorPropertyTemplate.h

@@ -318,6 +318,13 @@ class vtkSMVectorPropertyTemplate
       this->Property->Modified();
       this->ClearUncheckedElements();
       }
+    else if (this->Property->GetRepeatable())
+      {
+      this->Values.clear();
+      this->Initialized = true;
+      this->Property->Modified();
+      this->ClearUncheckedElements();            
+      }
     }
 
   //---------------------------------------------------------------------------