Propagate NoEvents from nested tubes

[sneakers-the-rat]

Fix: #162
Fix: #239

Interpret a None return value from a nested tube as a NoEvent in a parent.

This is the gentlest way to do this, without changing the default behavior of tubes to return NoEvent when no events made it to the return node (which we could do later, for consistency's sake) - this keeps the output of a runner falsy when no events are emitted.

two line change, the rest is docs and tests. could have reused existing tubes for this, but wanted to have a specific test for this, so it's more verbose than it could have been

edit: it turns out we needed a bit more, to actually propagate the signals from the return node -> tube node. Normally return nodes have no signals, because their values can't be returned within the tube (they are a "level shift" that brings events up to the calling scope). However when they are used in nested tubes, the return values become signals available in the parent tube. So by correctly inferring those and using those for NoEvent, we are basically fixing unimplemented but implied features of the tube node without adding special cases.

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📚 Documentation preview 📚: https://noob--245.org.readthedocs.build/en/245/

[codspeed-hq]

Merging this PR will not alter performance

✅ 8 untouched benchmarks
⏩ 8 skipped benchmarks¹

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_{Comparing issue-162-239 (2963430) with main (5c236d7)}

Footnotes

1. 8 benchmarks were skipped, so the baseline results were used instead. If they were deleted from the codebase, click here and archive them to remove them from the performance reports. ↩

[coveralls]

coverage: 84.631% (+0.04%) from 84.589% — issue-162-239 into main

[sneakers-the-rat]

maybe this should just be empty, no point in just always emitting NoEvent as if there was something to subscribe to