trivial: fix assignment-from-none pylint warning (#52732)

pylint.rc

@@ -9,7 +9,6 @@ disable=
     abstract-method,
     arguments-differ,
     assignment-from-no-return,
-    assignment-from-none,
     attribute-defined-outside-init,
     broad-except,
     consider-using-dict-comprehension,

wcs/workflows.py

@@ -1898,7 +1898,7 @@ class WorkflowStatusItem(XmlSerialisable):
         return label
 
     def get_line_details(self):
-        return None
+        return ''
 
     def render_list_of_roles(self, roles):
         return self.parent.parent.render_list_of_roles(roles)