fix: Pick Template BOM if variant BOM absent in WO popup from SO

[marination]

Closes #30655

Issue:

• BOM field stays blank if no Variant BOM is found in WO pop-up via Sales Order. Can't manually select BOM either due to filter
  

Fix:

• Pull BOM from template if variant BOM does not exist
  

[codecov]

Codecov Report

Merging #31353 (fafdaff) into develop (b8f728a) will decrease coverage by 0.18%.
The diff coverage is 92.30%.

@@             Coverage Diff             @@
##           develop   #31353      +/-   ##
===========================================
- Coverage    63.55%   63.37%   -0.19%     
===========================================
  Files          984      984              
  Lines        67555    67556       +1     
===========================================
- Hits         42936    42812     -124     
- Misses       24619    24744     +125     

Impacted Files	Coverage Δ
erpnext/stock/get_item_details.py	80.62% <90.00%> (+0.02%)	⬆️
erpnext/selling/doctype/sales_order/sales_order.py	80.47% <100.00%> (+0.36%)	⬆️
...unts/report/purchase_register/purchase_register.py	34.21% <0.00%> (-42.77%)	⬇️
...rial_no_valuation/incorrect_serial_no_valuation.py	59.64% <0.00%> (-26.32%)	⬇️
...e_sales_analytics/supplier_wise_sales_analytics.py	63.93% <0.00%> (-22.96%)	⬇️
...ext/accounts/report/balance_sheet/balance_sheet.py	36.36% <0.00%> (-21.82%)	⬇️
...e_purchase_register/item_wise_purchase_register.py	57.42% <0.00%> (-19.81%)	⬇️
erpnext/accounts/doctype/bank/bank.py	71.42% <0.00%> (-14.29%)	⬇️
...em_wise_sales_register/item_wise_sales_register.py	50.90% <0.00%> (-10.46%)	⬇️
...saction/incorrect_balance_qty_after_transaction.py	88.37% <0.00%> (-9.31%)	⬇️
... and 30 more