bug(graph): computed-condition If stalls when its branch reaches an empty Iterate through a For #9918

Summary

A session stalls when an If has a computed condition and its branch reaches an empty Iterate only through a For's final output. next() returns None, is_complete() stays False, and no error is raised, so the queue item hangs.

Reproduction

Nodes (the test nodes are from tests/test_nodes.py):

• src: CollectionConcatInvocation(first=[]) → A: IterateInvocation (empty)
• A.item → for: ForInvocation.collection. Its body is for.item → body: Mark("b:") → ret: ForReturnInvocation, linked to for.
• for.output_collection → mid: AnyTypeTestInvocation → if.true_input
• fallback: AnyTypeTestInvocation(value="fb") → if.false_input
• c: FixedCollectionTestInvocation(value=True), with c.condition → if.condition
• if → collect: CollectInvocation

Run it with GraphExecutionState on main (seen at ac237a1, on both schedulers).

Observed: only src and c run, and then the session stalls with collect never produced.

Expected: the session completes and collect gets [], because the If's only frame axis is empty. Two close relatives also stall on main but complete this way once #9789 is merged: the same graph without the For (A.item → mid → if.true_input), and the literal-condition version with the For.

A variant stalls too, on main and with #9789: A.item additionally feeds c.value, so the condition is per item.

Notes

A fix tried in #9789 broke a shape that main completes, so the stall is documented there as a known limitation instead.

Found during review of #9789.