Improve NotNull/NotEmpty validator messages

[oskogstad]

Check for use of

• Null
• Empty
• NotNull
• NotEmpty
  with When( ... ) clauses, inn all validators

Explained here:
#1110 (comment)

Instead of:

        RuleFor(x => x.TransmissionId)
            .Null()
            .When(x => x.ActivityType == DialogActivityType.Values.DialogOpened);

make it:

        RuleFor(x => x.TransmissionId)
            .Null()
            .When(x => x.ActivityType == DialogActivityType.Values.DialogOpened)
            .WithMessage($"{{PropertyName}} must be empty when {nameof(NotificationConditionQuery.ActivityType)} is {DialogActivityType.Values.DialogOpened}.");