comparison operations with a scalar null and decimal array panics

[kmitchener]

Describe the bug
A clear and concise description of what the bug is.

This test fails when given decimals instead of floats:'sql::expr::comparisons_with_null_lt' with error

 panicked at 'called `Result::unwrap()` on an `Err` value: "ArrowError(ExternalError(Internal(\"Cannot convert Decimal128(None,2,1) to i128\"))) ...

To Reproduce
Steps to reproduce the behavior:

You can reproduce with the following:

❯ select a < null from (values (1.1::decimal)) as t(a);
ArrowError(ExternalError(Internal("Cannot convert Decimal128(None,2,1) to i128")))

but interestingly, the scalar / scalar version of that work fine:

❯ select null < 1.1::decimal;
+---------------------------------+
| NULL < Decimal128(Some(11),2,1) |
+---------------------------------+
|                                 |
+---------------------------------+
1 row in set. Query took 0.002 seconds.

Expected behavior
A clear and concise description of what you expected to happen.

Additional context
Add any other context about the problem here.