Confirm semantics of "zero values" when deserializing

[ivnsch]

Given the API's behavior on receiver side ("zero value" fields have to be always mapped to "missing key") and some tests I did deserializing "zero value" fields(like on_complete), it can be likely assumed that the API always uses "missing key" to represent "zero value", and that when deserializing we should interpret "missing key" depending on context (are we expecting a "zero value"? then it's a "zero value", otherwise the field really doesn't exist), but we should confirm, either testing on a per-field basis or with a specification of the node's mapping policy / inspecting the code base.

The API documentation is not helpful, as it states to use "zero values", which is incorrect:
https://developer.algorand.org/docs/reference/transactions/
https://developer.algorand.org/docs/reference/teal/specification/#oncomplete

Related documentation issues (to date unaddressed):
algorandfoundation/docs#454
algorandfoundation/docs#415

We also should resolve possible ambiguities around "zero values" by specifying what they are: in some cases it's not obvious, e.g. an Address: is it an empty array or an array with 32 bytes set to 0? Is a struct with all the fields set to "zero value" a "zero value" (I remember that this was the case in the Java SDK)? etc.

[ivnsch]

Addressed by recent fixes. E.g. a non-existent application schema is interpreted as a schema with 0 values if the context indicates that a schema is expected. This is not ideal, but there's nothing that can be done on our side.