allow comparer to be omitted in EqualityExtensions

[SimonCropp]

it would seem the intent of AllAreTheSame is to allow people to omit passing a comparer ?

given the defaulting of the comparer to a value

comparer ??= EqualityComparer<TValue?>.Default;

but the api does not allow null to be passed?

if this is not the intent, i will do a PR that removed the defaulting