fix: check null values in is_cancelled patch (#30594) (#30595)

(cherry picked from commit bb875fe217)

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

erpnext/patches/v12_0/update_is_cancelled_field.py

@@ -20,7 +20,7 @@ def execute():
 			"""
 				UPDATE `tab{doctype}`
 				SET is_cancelled = 0
-				where is_cancelled in ('', NULL, 'No')""".format(
+				where is_cancelled in ('', 'No') or is_cancelled is NULL""".format(
 				doctype=doctype
 			)
 		)