Handle null objects correctly in emulate_bindings.

Note that fixing this opens up a bug compat issue, described in
comment #7 in [bug 5900].

lib/modules/Sql.pmod/sql_util.pmod

@@ -49,6 +49,12 @@ string emulate_bindings(string query, mapping(string|int:mixed)|void bindings,
 	lambda(mixed m) {
 	  if(zero_type(m))
 	    return "NULL";
+	  if (objectp (m) && m->is_val_null)
+	    // Note: Could need bug compatibility here - in some cases
+	    // we might be passed a null object that can be cast to
+	    // "", and before this it would be. This is an observed
+	    // compat issue in comment #7 in [bug 5900].
+	    return "NULL";
 	  if(multisetp(m))
 	    return sizeof(m) ? indices(m)[0] : "";
 	  return "'"+(intp(m)?(string)m:my_quote((string)m))+"'";