fix(comparison): wrap isnull equality check in parens

[gforsyth]

Description of changes

We wrap arguments to a sge.Equals in parens if the inputs are
themselves binary operations. There are other operations we will have
to wrap -- for now I'm just adding isNull to the list, but I imagine
this will grow, possibly on a backend-by-backend basis.

I'm going to investigate the mssql and oracle failures but they are
unrelated to this PR (they fail the added test with or without the parentheses)

Issues closed

• Resolves bug: missing parens in SQL with IS NULL #8346

[gforsyth]

snowflake and bigquery are passing the new test:

🐚 pytest ibis/backends/tests/test_generic.py -k isnull -m "bigquery or snowflake"
================================ test session starts ================================
platform linux -- Python 3.10.13, pytest-8.0.0, pluggy-1.4.0
Using --randomly-seed=4074414150
benchmark: 4.0.0 (defaults: timer=time.perf_counter disable_gc=False min_rounds=5 min_time=0.000005 max_time=1.0 calibration_precision=10 warmup=False warmup_iterations=100000)
rootdir: /home/gil/github.com/ibis-project/ibis
configfile: pyproject.toml
plugins: xdist-3.5.0, cov-4.1.0, mock-3.12.0, snapshot-0.9.0, randomly-3.15.0, hypothesis-6.98.3, benchmark-4.0.0, repeat-0.9.3, clarity-1.0.1, httpserver-1.0.8
collected 3440 items / 3438 deselected / 2 selected                                 

ibis/backends/tests/test_generic.py ..                                        [100%]

======================== 2 passed, 3438 deselected in 38.28s ========================

[cpcloud]

This has to be monkeypatched in to avoid altering the default backend for other tests

[gforsyth]

ARGH

I need to write a pre-commit check for this