Fix Nullable<bool>.ToString() conversion

[ranma42]

This PR updates translation of bool? expressions in 2 ways:

• null is now converted into the empty string, matching the usual C# behavior
• expressions are assumed to be nullable (they were previously assumed to be non-nullable)

Fixes #33941 for SQLite.
Contributes to #33942.

[maumar]

There are some product changes here. Consider splitting them out to make this pr test only, alternatively pull in the product changes for #33938 into one pr and update the PR description. Changes look good though.

[ranma42]

@maumar I moved the test updates and dataset changes in

• Improve some null-related tests #34019

I am not 100% sure if this is the "product changes"/all of them; I did not include the tests that were being added as I thought that they might belong to the specific fix/changeset.
I rebased this PR on top of that and marked it as draft as it is basically dependent on it.

Note that in order to ensure that the test suite passes, I disabled ToString_boolean_property_nullable in the PR the base PR and re-enabled it here.

EDIT: I also updated the title and first message of the PR to make it clearer that this one takes care of the fixing, while the other one only improves the tests.

[maumar]

@ranma42 huge apologies! I wanted to resolve conflicts in #34014 before approval and merge, and I accidentally force pushed into this branch instead (auto-complete >.<) - please tell me you still have these changes locally on your machine...

[ranma42]

@ranma42 huge apologies! I wanted to resolve conflicts in #34014 before approval and merge, and I accidentally force pushed into this branch instead (auto-complete >.<) - please tell me you still have these changes locally on your machine...

yup :) rebased and pushed this one 👍

also, in case it happens with your own commits (and/or with branches you have not replicated elsewhere, remember that git defaults to keeping commits around for 90 days; git reflog is very effective in saving you from this kind of mistakes

[maumar]

phew! Thanks making the PR (twice! ;) ) @ranma42