Fix build with Clang 19

[vient]

mfast315128adcd459/p/include/mfast/ext_ref.h:131:40: error: no member named 'optional' in 'ext_cref<mfast::decimal_cref, tuple<type-parameter-0-0, type-parameter-0-1>, type-parameter-0-2>'
  131 |   bool present() const { return !this->optional() || base_.present(); }
      |                                  ~~~~  ^
1 error generated.

Guess other places like this may fail too so patched them as well.