fix: typeerror on Sales Order analysis report

[ruthra-kumar]

If, for some reason delivery date is missing in Sales order, TypeError is thrown on Sales Order Analysis report with 'Group by Sales Order' enabled. Reporting 'null' for delay field is better than throwing typeerror which makes the report usable.

[codecov]

Codecov Report

Merging #34048 (00a4191) into develop (9d8502f) will decrease coverage by 0.03%.
The diff coverage is 12.50%.

Additional details and impacted files

@@             Coverage Diff             @@
##           develop   #34048      +/-   ##
===========================================
- Coverage    63.89%   63.86%   -0.03%     
===========================================
  Files          818      817       -1     
  Lines        59286    59327      +41     
===========================================
+ Hits         37878    37890      +12     
- Misses       21408    21437      +29     

Impacted Files	Coverage Δ
...eport/loan_interest_report/loan_interest_report.py	0.00% <0.00%> (ø)
...eport/sales_order_analysis/sales_order_analysis.py	79.38% <0.00%> (ø)
erpnext/stock/doctype/stock_entry/stock_entry.py	84.13% <ø> (ø)
erpnext/www/shop-by-category/index.py	0.00% <0.00%> (ø)
erpnext/projects/doctype/timesheet/timesheet.py	78.48% <100.00%> (+0.17%)	⬆️
.../setup/setup_wizard/operations/install_fixtures.py	90.90% <100.00%> (-0.22%)	⬇️
...pnext/setup/setup_wizard/operations/taxes_setup.py	90.90% <100.00%> (+0.06%)	⬆️
...ales_order/payment_terms_status_for_sales_order.py	80.99% <0.00%> (-4.33%)	⬇️
erpnext/stock/doctype/serial_no/serial_no.py	89.20% <0.00%> (-0.24%)	⬇️
...ccounts/doctype/payment_request/payment_request.py	67.98% <0.00%> (-0.11%)	⬇️
... and 8 more