Empty flattens cannot be used as side inputs.

{{with beam.Pipeline() as p:
main = p | "CM" \>\> beam.Create([])
side1 = p | "C1" \>\> beam.Create([])
side2 = p | "C2" \>\> beam.Create([])
side = (side1, side2) | beam.Flatten()
res = main | beam.Map(lambda x, side: x, beam.pvalue.AsList(side))
}}
results in
{{
File "/Users/robertwb/Work/beam/incubator-beam/sdks/python/apache_beam/runners/direct/evaluation_context.py", line 84, in add_values
assert not view.has_result
}}

Imported from Jira [BEAM-3393](https://issues.apache.org/jira/browse/BEAM-3393). Original Jira may contain additional context.
Reported by: robertwb.

Reproduce the minimal Python pipeline from the issue, then inspect sdks/python/apache_beam/runners/direct/evaluation_context.py around add_values and the failing assertion. Done means an empty Flatten can be used as an AsList side input without triggering the assertion; the payload names no test file, so add or locate a regression test for this case.

Written by the indexing model from the issue text.