[blob] Change null object memory mode to DUPLICATE

We never rely on that being equal to readonly.  Just not being
writable.  Maybe not even that given that the object is inert.

In prep for next commit, using default null pool.

src/hb-blob.cc

@@ -59,7 +59,7 @@ DEFINE_NULL_INSTANCE (hb_blob_t) =
 
   nullptr, /* data */
   0, /* length */
-  HB_MEMORY_MODE_READONLY, /* mode */
+  HB_MEMORY_MODE_DUPLICATE, /* mode */
 
   nullptr, /* user_data */
   nullptr  /* destroy */