Add handling for empty downstream sources

[mdemoret-nv]

Previously, we supported source->node->[nothing] where a source object didnt have any downstream sinks to write to. In that situation, the object was just discarded on writing. This fixes the edge functionality to replicate the previous system.

[ryanolson]

I feel like now is the time to enforce a null channel or a null cap.

Nodes with dangling outputs should throw an error and not start.

Nodes with dangling output can be null terminated which is the equivalent to dropping the data.

If the Morpheus stages, if you want this behavior by default you can opinionated it there.

[codecov]

Codecov Report

Merging #279 (7a6c6ef) into branch-23.01 (f526aff) will increase coverage by 0.05%.
The diff coverage is 83.33%.

Additional details and impacted files

@@               Coverage Diff                @@
##           branch-23.01     #279      +/-   ##
================================================
+ Coverage         72.92%   72.97%   +0.05%     
================================================
  Files               381      381              
  Lines             13103    13117      +14     
  Branches            991      991              
================================================
+ Hits               9555     9572      +17     
+ Misses             3548     3545       -3     

Flag	Coverage Δ
cpp	68.73% <83.33%> (+0.01%)	⬆️
py	41.03% <83.33%> (+0.11%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
cpp/mrc/src/tests/segments/common_segments.cpp	100.00% <ø> (ø)
cpp/mrc/src/tests/test_pipeline.cpp	97.23% <ø> (ø)
python/mrc/_pymrc/include/pymrc/node.hpp	61.22% <ø> (ø)
python/mrc/_pymrc/src/segment.cpp	82.11% <ø> (ø)
cpp/mrc/include/mrc/node/sink_properties.hpp	77.41% <66.66%> (-2.59%)	⬇️
cpp/mrc/include/mrc/node/source_channel_owner.hpp	90.90% <66.66%> (-9.10%)	⬇️
cpp/mrc/include/mrc/node/sink_channel_owner.hpp	100.00% <100.00%> (ø)
cpp/mrc/include/mrc/node/source_properties.hpp	75.00% <100.00%> (+5.30%)	⬆️
python/mrc/_pymrc/src/operators.cpp	83.87% <0.00%> (+2.41%)	⬆️
python/mrc/_pymrc/src/system.cpp	64.10% <0.00%> (+2.56%)	⬆️
... and 1 more

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[mdemoret-nv]

/merge