[Python] Fail WindowInto when there are no inputs to it

[riteshghorse]

It has been observed that pipelines get stuck when there are no input PCollections to a beam.WindowInto. No logs are observed in the console and the only option left is to force terminate jobs. This PR catches that case and hard fails the pipeline at submission time.

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[codecov]

Codecov Report

Merging #28946 (bdb2298) into master (2bfcb9f) will decrease coverage by 33.78%.
Report is 80 commits behind head on master.
The diff coverage is 0.00%.

@@             Coverage Diff             @@
##           master   #28946       +/-   ##
===========================================
- Coverage   72.16%   38.39%   -33.78%     
===========================================
  Files         686      686               
  Lines      101560   101630       +70     
===========================================
- Hits        73294    39017    -34277     
- Misses      26687    61034    +34347     
  Partials     1579     1579               

Flag	Coverage Δ
python	30.00% <0.00%> (-52.62%)	⬇️

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Files	Coverage Δ
sdks/python/apache_beam/runners/common.py	68.21% <0.00%> (-20.50%)	⬇️

... and 312 files with indirect coverage changes

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[riteshghorse]

Run Python 3.8 PostCommit

[riteshghorse]

R: @tvalentyn

[github-actions]

Stopping reviewer notifications for this pull request: review requested by someone other than the bot, ceding control

[robertwb]

FWIW, I think this should be fixed in the service rather than adding a special case to the SDK (Python only, which won't be out for 6 weeks or so).

[robertwb]

A good place to validate this might be https://github.com/apache/beam/blob/release-2.51.0/sdks/python/apache_beam/runners/common.py#L1895

We could have a mapping of known URNs -> expected input/output cardinality (if any), and flag violations. (We could ensure the inputs/outputs actually exist.)

p | beam.WindowInto(...) should also be an error right there (checked in WindowInto's expand method).