fix: Fix KeyError in vertical_block

This now mimics the way this is (safely) done in the sequence module [1].

References:
- [1] 5f94a082ce/common/lib/xmodule/xmodule/seq_module.py (L657-L658)

common/lib/xmodule/xmodule/vertical_block.py

@@ -66,7 +66,9 @@ class VerticalBlock(SequenceFields, XModuleFields, StudioEditableBlock, XmlParse
                     usage_key=self.location),  # pylint: disable=no-member
             if 'username' not in child_context:
                 user_service = self.runtime.service(self, 'user')
-                child_context['username'] = user_service.get_current_user().opt_attrs['edx-platform.username']
+                child_context['username'] = user_service.get_current_user().opt_attrs.get(
+                    'edx-platform.username'
+                )
 
         child_blocks = self.get_display_items()