[IDP-976] Post QA fixes

[asimmon]

Changes:

csharp_style_allow_blank_line_after_token_in_arrow_expression_clause_experimental set to true, because we use both styles:

MyMethod() =>
    { expression (usually a multi-line expression) }

// AND

MyMethod()
    => { expression (usually a single-line expression }

IDE0200 suggests to use method groups instead of lambda expressions, but there's a bug. Sometimes it shows the warning but it is impossible to use a method group. See example where we capture a variable and we still get a warning. Nullables also play a role in this equation, but the warning should not have been there as we capture an external variable: