Handle return type of 'null' expressions

[andrei-ungureanu-uipath]

When a precompiled value is created, determining the ReturnType of an expression may throw an exception for certain values like null and () => null because there needs to be an implicit cast on the SemanticModel to figure out the type. In these scenarios, the Type property will be null and we should fallback to the ConvertedType property as this one is populated when it cannot be inferred without casting.

[aoltean16]

These are both design-only features, as they're only called when typing into an expression textbox

[github-actions[bot]]

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