dbsp: Fix hidden dependency on vec-based batches in input_upsert.

[blp]

The vec-based batches set keys, values, and weights to their default values when their builders accept tuples via Builder::push or Builder::push_vals, which take mutable references. The code for input_upsert accumulates updates to keys in a key_updates vector, and then adds them to a builder via extend, which internally uses Builder::push and thus clears all the updates that were added to have weight 0.

Before this commit, that code didn't later clear the key_updates vector, which meant that anything added later was appended to a bunch of zeroed tuples. This was benign because, the next time key_updates was used, it was first consolidated, which meant that all the zero-weight tuples were dropped.

This failed with the file-based batches, which don't have any special optimizations for mutable references and thus don't zero anything, which meant that nothing in key_updates ever got cleared. This commit fixes the problem by properly clearing key_updates to empty after using it.

Is this a user-visible change (yes/no): ___