simple_* methods handle `None` incorrectly for key values

This issue has been migrated from [#14157](https://github.com/matrix-org/synapse/issues/14157).

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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:

https://github.com/matrix-org/synapse/blob/b4ec4f5e71a87d5bdc840a4220dfd9a34c54c847/synapse/storage/database.py#L1304-L1310

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).