[bug]: For loop over an empty Iterate item fails with NodeInputError #9819

Is there an existing issue for this problem?

• I have searched the existing issues

Version number

main at aa46003 (7.0.0-alpha.2-post); also on #9789

What happened

Feed each item of an Iterate into a For loop's collection. If any item is an empty list, the session fails with NodeInputError: Node … has invalid incoming input for a when the loop body runs. No If is involved.

What you expected to happen

The empty item's For should run zero iterations and give an empty output_collection. The non-empty items should run normally. Here the result should be [[11, 12], []].

How to reproduce the problem

g = Graph()
g.add_node(CollectionConcatInvocation(id="source", first=[[1, 2], []]))
g.add_node(IterateInvocation(id="iterate"))
g.add_node(ForInvocation(id="for"))
g.add_node(AddInvocation(id="body", b=10))
g.add_node(ForReturnInvocation(id="return"))
g.add_node(CollectInvocation(id="collect"))
g.add_edge(create_edge("source", "collection", "iterate", "collection"))
g.add_edge(create_edge("iterate", "item", "for", "collection"))
g.add_edge(create_edge("for", "item", "body", "a"))
g.add_edge(create_edge("body", "value", "return", "output"))
g.add_edge(create_loop_linkage("for", "return"))
g.add_edge(create_edge("for", "output_collection", "collect", "item"))

Additional context

The trace is source, iterate, iterate, for, body. A body execution is created for the empty item's For (index -1) without its a input.

Found while reviewing #9789.