When using the null propagation operator on a nullable sealed type, trying to access undefined properties trigger a warning instead of an error

**Bicep version**
Bicep CLI version 0.39.26 (1e90b06e40)

**Describe the bug**
When I have a user-defined type that has the `sealed` decorator attributed to it. Attempting to to access a property that is not defined in by the type returns a warning instead of an error.

It would be more sensible to return an error as the expectation from the `sealed` decorator is that any property accessed on a typed symbol should be defined in the type definition. Whether it is a conditional access or not.

**To Reproduce**
```bicep
@sealed()
type sealedType = {
propertyA: string
}

param mySealedParam sealedType?

var shouldFail = mySealedParam.?propertyB
```

A warning is raised for `propertyB` instead of an error.

Start with the provided Bicep reproduction using a sealed nullable user-defined type and conditional access to an undefined property. Trace the diagnostic produced for propertyB; done means the undefined property is reported as an error rather than a warning, including when null propagation is used.

Written by the indexing model from the issue text.