Fix: Skip expansion of mapped task group when list is empty (2.5.1+affirm4)

[andresvelasquez2-affirm]

Summary

Ports upstream fix apache/airflow#30084 to our 2.5.1 fork, and bumps the fork version to 2.5.1+affirm4.

Problem

Expanding a mapped task group fails when the input list is empty and a downstream task references a mapped index via XCom pull of that group. In that case, the entire task group should be skipped — instead, the scheduler enters a crash loop.

Root cause: in TaskInstance.get_relevant_upstream_map_indexes, ancestor_map_index is calculated as self.map_index * ancestor_ti_count // ti_count. When a task group is expanded with an empty list, ti_count is 0, causing a ZeroDivisionError.

Fix

Move the early-return guard before the division and broaden it from if ti_count is None to if not ti_count, so both None and 0 short-circuit to returning None (which callers already treat as "no partial mapping — use the whole value / all upstreams").

Changes

• airflow/models/taskinstance.py — reorder and broaden the ti_count guard in get_relevant_upstream_map_indexes.
• tests/ti_deps/deps/test_trigger_rule_dep.py — add test_upstream_in_mapped_group_when_mapped_tasks_list_is_empty regression test.
• setup.py — bump fork version 2.5.1+affirm3 → 2.5.1+affirm4.

Compatibility

Signature of get_relevant_upstream_map_indexes is unchanged. Both call sites (airflow/models/xcom_arg.py and airflow/ti_deps/deps/trigger_rule_dep.py) already handle a None return.

Test plan

• New regression test test_upstream_in_mapped_group_when_mapped_tasks_list_is_empty passes.
• Existing trigger rule tests continue to pass.