Form logging doesn't include

Here: https://github.com/NickCraver/StackExchange.Exceptional/blob/main/src/StackExchange.Exceptional.AspNetCore/AspNetCoreExtensions.cs#L156-L159 isn't logging a form key that has no values, this tripped us up when debugging today. Needs handling for 0 values and adding a single pair as null.

cc @m0sa 