Firestore: Treat 'None' as EOF in 'Watch.on_snapshot'.

[tseaver]

See:
#8650 (review)

[plamut]

LGTM (the code changes)

watch.py needs to be blackened, though, and I'm not sure about the API core failure (unrelated?)

Update: The API core failure was apparently flakiness.

[plamut]

Something weird is going on, I removed and re-added the force run label, the label is there, but there is no record of that on the PR timeline, and the yoshi-kokoro bot does not see it.

[crwilcox]

I cleaned out firestore to see if that helps. forcing a ci run.

[tseaver]

The Kokoro jobs for Bigtable and Natural Language both fail due to a Kokoro-internal glitch:

Cannot check out non-mergeable pull request as if merged.

[plamut]

@tseaver Is there anything we can do to get around the glitch?

[tseaver]

@plamut

Is there anything we can do to get around the glitch?

I'm actually worried that there is something more sinister at play.

@crwilcox Can you double-check that the clean-out really worked in the CI project? E.g.:

documents = []
for collection in client.collections():
    documents.extend(collection.list_documents()
assert len(documents) == 0

[tseaver]

I have added a commit to make the system test output verbose, so that we can figure out which test is hanging.

[tseaver]

@crwilcox I'm expecting this Kokoro run to time out again: can you post the hung systest output when it does? Or maybe you have the keys to see the output even before it times out?