Correct nullability for set operations

[roji]

Fixes #18135

@smitpatel some minor comments:

• Technically we should also make the outer EntityProjectionExpression itself nullable when one of the inner ones is nullable, but we currently can't check if an EntityProjectionExpression is nullable or not - we could expose IsNullable for this purpose. This doesn't seem to matter in practice since EntityProjectionExpression's nullability is only consulted in BindProperty, but for set operations _propertyExpressionsCache is already populated correctly. Let me know if you want me to do this and expose IsNullable.
• In a perfect world, isNullableProjection should drill down into the expression to really understand whether it's nullable or not. Currently we just assume nullable except for some very basic cases.
• I currently duplicated isNullableProjection from ColumnExpression, we should DRY this at some point.

[smitpatel]

EntityProjection is marked as nullable only if it is mapped to a table. The source table can project out nulls so when generating ColumnExpression make them nullable. If the entityProjection is not mapped to a table then nullable flag is unnecessary. The backing propertyExpressions should already be nullable if required. May be we could decorate TableExpressionBase as nullable but breaking change.

1. So no, we are not exposing EntityProjection.IsNullable or marking EntityProjection as nullable when passing propertyExpressions.
2. Low value, Some perf benefit.
3. May be.

[roji]

The backing propertyExpressions should already be nullable if required

Yeah, I see that - just a bit uncomfortable about having EntityProjection lying about its nullability, in case we start looking at it in some place in the future etc.

[smitpatel]

EntityProjection does not lie about anything because it does not tell you its nullability.

[roji]

EntityProjection does not lie about anything because it does not tell you its nullability.

Right, but it contains an incorrect setting about itself, in case we ever need to rebind a property or similar. I understand it's irrelevant in the current code/design though.

[smitpatel]

What is rebinding a property? If you pass a table then tell if it can be null. Purely metadata to create columnExpression if not created already. There is no setting which is incorrect.

[roji]

Never mind, am just leaving it as-is.

[smitpatel]

If you think there is a bug or design issue somewhere then please explain in detail what scenario is not working or why it is an issue.

cc: @ajcvickers

[roji]

Apologies, was not suggesting there was a bug in the design or anything... There's no bug here, just a possible matter of future-proofing it by making sure that EntityProjections always has the correct _nullable value internally, which I wanted to signal. It's valid to skip this if you don't think it's worthwhile.