Handle blank signatories in migrate_edx_data (#3125)

Author: rachellougee

courses/management/commands/migrate_edx_data.py

@@ -193,9 +193,18 @@ def _migrate_course_runs(self, conn, options):
                     continue
 
                 signatories = self._get_signatories(
-                    row.get("signatory_names", []), use_default_signatory
+                    row.get("signatory_names") or [], use_default_signatory
                 )
 
+                if not signatories and row.get("mitxonline_course_id") is None:
+                    self.stdout.write(
+                        self.style.ERROR(
+                            f"No valid signatories found with names {row.get('signatory_names')} for course "
+                            f"{row.get('course_readable_id')}, skipping it."
+                        )
+                    )
+                    continue
+
                 (course, course_created) = self._create_course(row)
 
                 if course_created: