Fixes crash in Assistant.

Root cause of the crash was committing a query without first having
pended a query.

This can occur in a notifications flow or a device actions flow in
which an interaction did not originate from the UI.

We rely on pending/committing the query in the interaction model to
clear the previous query from the UI. The fix, in this case, was just
to pend an empty query so that we have something to commit.

Bug: b:111923411
Change-Id: I53848f856375cf220538738634e7543a455c0dcc
Reviewed-on: https://chromium-review.googlesource.com/1155153
Reviewed-by: Xiaohui Chen <xiaohuic@chromium.org>
Commit-Queue: David Black <dmblack@google.com>
Cr-Commit-Position: refs/heads/master@{#579271}
6 files changed