"" gives an empty string, while "".idup gives null

[simendsjo]

On 03.08.2011 15:49, bearophile wrote:
> simendsjo:
>
>> void main() {
>>       assert(is(typeof("") == typeof("".idup))); // both is immutable(char)[]
>>
>>       assert(""      !is null);
>>       assert("".idup !is null); // fails - s is null. Why?
>> }
>
> I think someone has even suggested to statically forbid "is null" on strings :-)
>
> Bye,
> bearophile

How should I test for null if not with "is null"? There is a difference 
between null and empty, and avoiding this is not necessarily easy or 
even wanted.
I couldn't find anything in the specification stating this difference.
So... Is it a bug?