Publish empty results in aggregator iff no more results are coming

Before this CL, the aggregator publishes an empty set of results only
when publishing burn in results - once burn in completes, results are
ALWAYS published, even if no sources have returned yet. After burn in
completes, empty results are filtered out.

This CL changes this behaviour so empty results are published if and
only if no more results are coming. This removes the aforementioned
burn-in logic, and allows the view to know when no more results will
come in. This means that the aggregator now publishes *an additional
empty set of results*.

At the moment, empty results are only used for category-filtered
searches to signify that no results were found. After this CL,
category-filtered searches with results will publish an additional time
with a set of empty results - effectively a no-op - and
category-filtered searches without results will remain the same as
before (publishing only one set of empty results).

As a result of this change, if all results complete before the burn in
period, all results are IMMEDIATELY published (along with another "empty
results" to signal that no more results are coming).

This will be used to signal to the view when we can show the "no
results" screen - and possibly stopping the loading animation in the
future.

Bug: b:348317154
Change-Id: I6a6a2100a712a3f1d0541c34ddc19f7c9d93a094
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5657840
Reviewed-by: Darren Shen <shend@chromium.org>
Commit-Queue: Michael Cui <mlcui@google.com>
Cr-Commit-Position: refs/heads/main@{#1321552}