Avoid divide-by-zero when there is no packing list

[arpitjain099]

Fixes #391.

lift_report_to_blocks and ship_report_to_blocks compute the success percentage as num_success_pallets / total_pallets * 100. When a ship runs with no packing list, total_pallets is 0 (or missing, in which case _safely_access returns None), so this raises ZeroDivisionError / TypeError and the ship fails - the traceback in the issue points at slack line 140.

Both report paths now guard the division: when total_pallets is falsy the percentage is reported as 100 (no pallets means nothing failed, so the existing :100: path renders "0 of 0 pallets ran successfully"). I went with 100 rather than 0 since an empty run has no failures; happy to switch to 0 or a "no pallets" message if you prefer.

Added test_ship_with_no_packing_list and test_lift_with_no_packing_list. I could not run the suite locally because forklift.models imports arcpy (ArcGIS Pro, not installable outside an ArcGIS environment), so I verified the percentage logic standalone (total=0 and total=None both yield 100; normal ratios unchanged) and py_compiled the changed files. The new tests follow the existing test_slack.py structure and exercise the guarded path.

[Copilot]

Pull request overview

This PR fixes a runtime failure in Slack report generation when shipping/lifting runs with no packing list (i.e., total_pallets == 0), preventing ZeroDivisionError/TypeError and allowing the scheduled ship→lift workflow to complete (Fixes #391).

Changes:

• Guard success-percentage division in lift_report_to_blocks and ship_report_to_blocks when total_pallets is falsy.
• Add unit tests covering ship/lift reports with total_pallets = 0 to ensure the guarded path doesn’t raise.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
src/forklift/slack.py	Adds a total_pallets guard so success percentage calculation won’t divide by zero when no packing list exists.
tests/test_slack.py	Adds tests to exercise ship/lift Slack report generation when total_pallets is 0.

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[stdavis]

Thanks for the contribution @arpitjain099! Do you use this project? If so, I'd love to hear about it.

[arpitjain099]

@stdavis Thanks for the quick review and merge! I don't use forklift directly - spotted the issue while browsing open bugs and the zero-division traceback looked like a clean fix. Cool project honestly, the pallet/warehouse metaphor for ETL is fun.

[stdavis]

Well thanks for the random contribution!