PR #7202 added empty-assistant filtering in `_query` so strict
providers (Moonshot, etc.) wouldn't 400 on history with blank
assistant entries. The streaming sibling `_query_stream` was
never updated, so DeepSeek Reasoner — which returns reasoning only
during tool calls, leaving serialized content as `""` — blew up with
`Invalid assistant message: content or tool_calls must be set` on
the next turn.
Hoisted the filter into a `_sanitize_assistant_messages` helper and
called it from both paths. Also widened the empty check to cover
`content == []`, which the original filter missed and which shows up
with providers that emit content as a list of parts.
55c1558686