fix: SQL error on fetching RM in production plan

[ankush]

Problem:
sql error on fetching RM from production plan

Steps to reproduce:

• Create new production plan
• pick any item with BOM
• Add material request warehouse
• get RM > enter any warehouse (try a group warehouse-like ALL WH)

You'll get SQL error because the warehouse JSON is being sent to query instead of a parsed list.

Root cause:
The list is getting passed by reference and compared using not which would ignore the referenced list if it's empty and create new one instead.

erpnext/erpnext/manufacturing/doctype/production_plan/production_plan.py

Lines 751 to 752 in 28d52c4

	if not warehouse_list:
	warehouse_list = []

Solution:
check the passed reference for None, not for falseness.
refactor and avoid passing by reference / mutations.