fix: Use fallback conversion factor while setting incoming rate for petty purchase

[marination]

• Purchase Invoices for petty items (that do not need an Item record) are allowed using Item Name field
• If a different UOM (different from stock UOM) is used in this case, conversion factor stays 0 and causes an error

 File "apps/erpnext/erpnext/controllers/buying_controller.py", line 33, in validate
    super(BuyingController, self).validate()
  File "apps/erpnext/erpnext/controllers/stock_controller.py", line 38, in validate
    super(StockController, self).validate()
  File "apps/erpnext/erpnext/controllers/accounts_controller.py", line 155, in validate
    self.set_incoming_rate()
  File "apps/erpnext/erpnext/controllers/buying_controller.py", line 302, in set_incoming_rate
    rate = flt(outgoing_rate * d.conversion_factor, d.precision("rate"))
TypeError: unsupported operand type(s) for *: 'float' and 'NoneType'

Fix:

• Fallback to 1 as conversion factor in set_incoming_rate for buying
• Selling will need a proper item, so this change is not needed there

[codecov]

Codecov Report

Merging #31529 (aa043fe) into develop (37f7801) will increase coverage by 0.06%.
The diff coverage is 100.00%.

@@             Coverage Diff             @@
##           develop   #31529      +/-   ##
===========================================
+ Coverage    64.33%   64.40%   +0.06%     
===========================================
  Files          965      965              
  Lines        65156    65156              
===========================================
+ Hits         41918    41963      +45     
+ Misses       23238    23193      -45     

Impacted Files	Coverage Δ
erpnext/controllers/buying_controller.py	87.85% <100.00%> (ø)
...value/warehouse_wise_item_balance_age_and_value.py	89.02% <0.00%> (-2.44%)	⬇️
erpnext/stock/doctype/item_price/item_price.py	92.98% <0.00%> (-1.76%)	⬇️
erpnext/portal/utils.py	27.69% <0.00%> (-1.54%)	⬇️
erpnext/crm/doctype/prospect/prospect.py	49.25% <0.00%> (-1.50%)	⬇️
erpnext/stock/report/stock_ageing/stock_ageing.py	94.71% <0.00%> (-0.97%)	⬇️
erpnext/support/doctype/issue/issue.py	64.28% <0.00%> (-0.96%)	⬇️
erpnext/crm/doctype/lead/lead.py	70.51% <0.00%> (-0.86%)	⬇️
erpnext/controllers/sales_and_purchase_return.py	91.41% <0.00%> (-0.75%)	⬇️
...t/accounts/report/general_ledger/general_ledger.py	67.57% <0.00%> (-0.35%)	⬇️
... and 18 more

[deepeshgarg007]

@Mergifyio backport version-13

[mergify]

backport version-13

✅ Backports have been created

• #31558 fix: Use fallback conversion factor while setting incoming rate for petty purchase (backport #31529) has been created for branch version-13