[C++][Acero][Python] Asof join does not detect null times

[hadrian-reppas]

Describe the bug, including details regarding any error messages, version, and platform.

The current asof join implementation does not check the null bitmap before indexing into the time column's value buffer. This creates matches that should not be possible. For example,

import pyarrow as pa

lhs = pa.table({
 "time": pa.array([None], type=pa.int64())
})
rhs = pa.table({
 "time": [0],
 "data": [True],
})

lhs.join_asof(rhs, "time", [], 0)

produces

 time data
0 null true

which implies that null equals 0. By manipulating the value buffer, we can get null to "equal" any integer:

lhs = pa.table({
 "time": pa.Array.from_buffers(
 pa.int32(),
 1,
 [pa.py_buffer(b"\x00"), pa.py_buffer(b"\xf0\x00\x00\x00")],
 ),
})

rhs = pa.table({
 "time": pa.array([0xf0], type=pa.int32()),
 "payload": ["abc"],
})

# lhs: rhs:
# time time payload
# 0 null 0 240 abc

result = lhs.join_asof(rhs, "time", [], 0)

# result:
# time payload
# 0 null abc -> non-null payload means null matched 240

This could be fixed by adding a null check in the GetTime function in time_series_util.cc, but it's not clear to me what the correct behavior should be.

Component(s)

C++

[hadrian-reppas]

This also seems to be an issue in by-keys:

lhs = pa.table({
 "time": [0],
 "key": [0],
})
rhs = pa.table({
 "time": [0],
 "key": pa.array([None], type=pa.int64()),
 "payload": ["abc"],
})

produces

 time key payload
0 0 0 abc

Hasher64::HashMultiColumn hashes nulls to 0 and the asof join does not handle collisions.