Checking if a string is null

[Manfred Nowak]

Frits van Bommel wrote

> But the fact of the matter is, 'T[] x = null;' reserves space for
> the .length and sets it to 0. If you have a suggestion for a
> different value to put there, by all means make it.

Suggestion:
After `T[] x= null;' `x.length == size_t.max' and `x.ptr == null', i.e.  
`size_t.max' will no more be a valid length for an array.

This is a hack to avoid some overhead in some places, but may introduce  
more overhead in other places.

Note: after `T[] x= null;' `x' holds an untyped array and so `y= x;' 
should be a legal assignment for every `y' declared as `U[] y;' for 
some type `U'---duck and run.

-manfred