fix: do not drop falsy shard keys in convert_points_update_operation - #1279

All eight branches of `RestToGrpc.convert_points_update_operation` gate the
shard key on truthiness:

    shard_key_selector = (
        cls.convert_shard_key_selector(model.upsert.shard_key)
        if model.upsert.shard_key
        else None
    )

`ShardKey` is `Union[StrictInt, StrictStr]`, so `0` and `""` are valid keys
that evaluate to False. They get silently replaced with `None`, and the
operation is routed by the default hashing rules instead of to the requested
shard. No error is raised on either side, so the write just lands somewhere
else.

Before, converting each operation type with `shard_key=0`:

    LOST UpsertOperation            shard_key_selector present=False
    LOST DeleteOperation            shard_key_selector present=False
    LOST SetPayloadOperation        shard_key_selector present=False
    LOST OverwritePayloadOperation  shard_key_selector present=False
    LOST DeletePayloadOperation     shard_key_selector present=False
    LOST ClearPayloadOperation      shard_key_selector present=False
    LOST UpdateVectorsOperation     shard_key_selector present=False
    LOST DeleteVectorsOperation     shard_key_selector present=False

    8/8 operations silently dropped the shard key
    control, shard_key=1 -> [number: 1]

After:

    OK   UpsertOperation            shard_key_selector present=True keys=[number: 0]
    ... (all eight)
    0/8 operations silently dropped the shard key

This is on the write path: `QdrantRemote.batch_update_points` calls
`RestToGrpc.convert_update_operation` for every operation when the client is
built with `prefer_grpc=True`, so a custom-sharded collection keyed on
integers loses shard 0 on every batch update.

The same conversion elsewhere in the file already uses the `is not None`
form, for example `convert_query_request`:

    shard_key_selector=(
        cls.convert_shard_key_selector(model.shard_key)
        if model.shard_key is not None
        else None
    ),

so this just brings the update path in line with it. The field is
`Optional[ShardKeySelector]` with a `None` default, and `None` is the only
value that should mean "not set".

Tested with `pytest tests/conversions/test_validate_conversions.py`
(22 passed). The new test fails on all eight operations without the
conversion.py change.