Fix GH-9905: constant() behaves inconsistent when class is undefined

Directly referring to a constant of an undefined throws an exception;
there is not much point in `constant()` raising

Fix GH-9905: constant() behaves inconsistent when class is undefined

Directly referring to a constant of an undefined throws an exception;
there is not much point in `constant()` raising a fatal error in this
case.

Closes GH-9907.

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