fix: can't cancel reconciliation that created new sr no

[ankush]

closes: #25390

Steps to reproduce:

1. Create a new serialized item (WITHOUT batches)
2. Create a stock reco and add serial nos for it via stock reco.
3. New serial nos will get created
4. Cancel the said stock reco. You'll get error that sr is undefined.

Root cause: stock reco was setting "0" as current_serial_no instead it should have been "" (empty string) or None.

Fix: set current_serial_no to empty string when none are found
patch: update stock reco item row's serial no to NULL where qty is 0.

Minor unrelated perf: N+1 query for unlinking inactive serial nos instead of N+2. (will become just 2 after frappe/frappe#15560 goes through)

[codecov]

Codecov Report

Merging #29393 (a6472d6) into develop (f5ed0c8) will increase coverage by 0.05%.
The diff coverage is 100.00%.

@@             Coverage Diff             @@
##           develop   #29393      +/-   ##
===========================================
+ Coverage    57.87%   57.92%   +0.05%     
===========================================
  Files         1091     1091              
  Lines        67894    67893       -1     
===========================================
+ Hits         39292    39329      +37     
+ Misses       28602    28564      -38     

Impacted Files	Coverage Δ
erpnext/stock/utils.py	67.57% <ø> (ø)
erpnext/controllers/stock_controller.py	90.65% <100.00%> (-0.05%)	⬇️
...ext/accounts/report/balance_sheet/balance_sheet.py	36.36% <0.00%> (-21.82%)	⬇️
...payroll/doctype/income_tax_slab/income_tax_slab.py	83.33% <0.00%> (-16.67%)	⬇️
...ctype/woocommerce_settings/woocommerce_settings.py	80.76% <0.00%> (-3.85%)	⬇️
...cial_statement/consolidated_financial_statement.py	88.81% <0.00%> (-1.60%)	⬇️
...ctype/accounting_dimension/accounting_dimension.py	64.34% <0.00%> (-1.56%)	⬇️
...e/period_closing_voucher/period_closing_voucher.py	88.23% <0.00%> (-1.48%)	⬇️
...rpnext/stock/report/stock_balance/stock_balance.py	79.16% <0.00%> (-0.60%)	⬇️
erpnext/stock/stock_ledger.py	85.42% <0.00%> (-0.56%)	⬇️
... and 16 more