fix bug with binary values not being saved to disk

For SDAI_Binary, STEPattribute::is_null() cleared the value instead of
merely reporting whether it was null, like it does with other types.

src/clstepcore/STEPattribute.cc

@@ -790,7 +790,6 @@ int STEPattribute::is_null()  const {
             return ( *( ptr.S ) == S_STRING_NULL );
 
         case BINARY_TYPE:
-            ptr.b->clear();
             return ptr.b->empty();
 
         case AGGREGATE_TYPE: