[iter] Use hb_declval() instead of Null() to get instance

I had used Null to make one of the bots happy before.  Not going
to bend to such demands anymore..

src/hb-iter.hh

@@ -30,7 +30,6 @@
 #include "hb.hh"
 #include "hb-algs.hh" // for hb_addressof
 #include "hb-meta.hh"
-#include "hb-null.hh"
 
 
 /* Unified iterator object.
@@ -155,7 +154,7 @@ template<typename T, typename B>
 struct _hb_is_iterable
 { enum { value = false }; };
 template<typename T>
-struct _hb_is_iterable<T, hb_bool_tt<true || sizeof (Null(T).iter ())> >
+struct _hb_is_iterable<T, hb_bool_tt<true || sizeof (hb_declval (T).iter ())> >
 { enum { value = true }; };
 
 template<typename T>

src/hb.hh

@@ -635,7 +635,7 @@ _hb_memalign(void **memptr, size_t alignment, size_t size)
 #include "hb-atomic.hh"	// Requires: hb-meta
 #include "hb-null.hh"	// Requires: hb-meta
 #include "hb-algs.hh"	// Requires: hb-null
-#include "hb-iter.hh"	// Requires: hb-algs hb-meta hb-null
+#include "hb-iter.hh"	// Requires: hb-algs hb-meta
 #include "hb-debug.hh"	// Requires: hb-algs hb-atomic
 #include "hb-array.hh"	// Requires: hb-algs hb-iter hb-null
 #include "hb-vector.hh"	// Requires: hb-array hb-null