Dummy streams should be pending, not empty

[palfrey]

Description

#2076 has some dummy streams both for the unused case for subscriber_channel (only used by Redis clusters) and for the MockPubSub in tests. Both of the dummy streams were empty i.e. they continually return a None which is bad because the rest of the code assumes None means "the other end disconnected, so error out". This results in a lot of

{"timestamp":"2026-02-13T17:31:23.617211Z","level":"ERROR","fields":{"message":"Error receiving message in RedisSubscriptionManager from subscriber_channel"},"target":"nativelink_store::redis_store","span":{"name":"redis_subscribe_spawn"},"spans":[{"name":"redis_subscribe_spawn"}]}

This PR makes both dummy streams pending ones i.e. they just sit there and output nothing, which is what we actually want.

Type of change

Please delete options that aren't relevant.

• Bug fix (non-breaking change which fixes an issue)

How Has This Been Tested?

bazel test //...

Checklist

• Updated documentation if needed
• Tests added/amended
• bazel test //... passes locally
• PR is contained in a single commit, using git amend see some docs

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This change is