Make completed_cells column nullable

Author: agupta01

jupyter_scheduler/orm.py

@@ -103,7 +103,7 @@ class Job(CommonColumns, Base):
     url = Column(String(256), default=generate_jobs_url)
     pid = Column(Integer)
     idempotency_token = Column(String(256))
-    completed_cells = Column(Integer)
+    completed_cells = Column(Integer, nullable=True)
     # All new columns added to this table must be nullable to ensure compatibility during database migrations.
     # Any default values specified for new columns will be ignored during the migration process.