simple_* methods handle `None` incorrectly for key values

[clokep]

Generally the simple_* methods do not handle None properly when used as a key-value (the exact arguments differ by method). See #14138 for real fallout from this.

simple_upsert_emulated_txn has special handling for null which should likely be abstracted:

synapse/synapse/storage/database.py

Lines 1304 to 1310 in b4ec4f5

	def _getwhere(key: str) -> str:
	# If the value we're passing in is None (aka NULL), we need to use
	# IS, not =, as NULL = NULL equals NULL (False).
	if keyvalues[key] is None:
	return "%s IS ?" % (key,)
	else:
	return "%s = ?" % (key,)

This is mostly a footgun that we could avoid by either:

1. Raising an exception (or asserting) when a key-value is None.
2. Automatically handling None and turning it into IS NONE (which is what simple_upsert_emulated_txn does).