prog: use consistent default values for conditional unions

We used to assume that the default value was the last, yet when it was
not specified in the serialized program, the first union option whose
condition is satisfied was chosen. Let's be consistent and use the last
value in both cases.

Also, remember that there's a case when there's no valid default value -
this happens when pkg/compiler wraps a conditional field into a union
with two conditional fields. Explicitly check for this case and assume
that, whatever value is set, is the correct default because in this
particular case the conditions of the two union options must be
mutually exclusive.

Fixes #6105.

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