fix(desk): guard owner/modified_by access in update_user_info (#35581)

* fix(desk): guard owner/modified_by access in update_user_info

Fixes a regression introduced by #35557

This updates the code to safely check for the presence of those
fields using `getattr` before adding them to the uservlist. This
preserves the intended upstream behavior while avoiding
runtime errors for virtual/single/transient documents.

* chore: cache getattr instead of calling it twice

* refactor: less code

---------

Co-authored-by: Ankush Menat <ankush@frappe.io>

frappe/desk/doctype/system_health_report/test_system_health_report.py

@@ -2,9 +2,10 @@
 # See license.txt
 
 import frappe
+from frappe.desk.form.load import getdoc
 from frappe.tests import IntegrationTestCase
 
 
 class TestSystemHealthReport(IntegrationTestCase):
 	def test_it_works(self):
-		frappe.get_doc("System Health Report")
+		getdoc("System Health Report", "System Health Report")

frappe/desk/form/load.py

@@ -481,8 +481,9 @@ def update_user_info(docinfo, doc=None):
 	users = set()
 
 	if doc:
-		users.add(doc.owner)
-		users.add(doc.modified_by)
+		for field in ("owner", "modified_by"):
+			if user := doc.get(field):
+				users.add(user)
 
 	users.update(d.sender for d in docinfo.communications)
 	users.update(d.user for d in docinfo.shared)