Unify absent sequences and records with empty ones

[jyasskin]

In many, but not all cases, an absent sequence or record argument is equivalent to an empty one. The spec should be clear about when that is or isn't the case. When they are equivalent, the sequence or record should be defaulted to empty in order to reduce the number of states the specification has to deal with.

For sequences, we can use =[] in the dictionary definition to set the default value. For records, I don't think ={} works, but we can do it the first time those members are read or by setting the initial value of the struct they're copied into.