bindnode doesn't calculate map `Length()` properly when properties are absent

Can be seen @ https://github.com/ipfs/go-graphsync/pull/332

Relevant bit of schema looks like:

type GraphSyncRequest struct {
  id                GraphSyncRequestID  (rename "ID")   # unique id set on the requester side
  root     optional Link                (rename "Root") # a CID for the root node in the query
  selector optional Any                 (rename "Sel")  # see https://github.com/ipld/specs/blob/master/selectors/selectors.md
  extensions        GraphSyncExtensions (rename "Ext")  # side channel information
  priority          GraphSyncPriority   (rename "Pri")  # the priority (normalized). default to 1
  cancel            Bool                (rename "Canc") # whether this cancels a request
  update            Bool                (rename "Updt") # whether this is an update to an in progress request
} representation map

So, using pointers for the 2 optional fields. But then when encoding one of these, with nil values, to dag-cbor I get:

a364426c6b7380645265717381a76249445020109ec634284a19a31e56de508bd0f063457874a063507269006443616e63f56455706474f4645273707380

Which, looks like this:

A3                                      # map(3)
   64                                   # text(4)
      426C6B73                          # "Blks"
   80                                   # array(0)
   64                                   # text(4)
      52657173                          # "Reqs"
   81                                   # array(1)
      A7                                # map(7)
         62                             # text(2)
            4944                        # "ID"
         50                             # bytes(16)
            2E039BD0020B4AE59E8983A201E5A777 # ".\x03\x9B\xD0\x02\vJ\xE5\x9E\x89\x83\xA2\x01\xE5\xA7w"
         63                             # text(3)
            457874                      # "Ext"
         A0                             # map(0)
         63                             # text(3)
            507269                      # "Pri"
         00                             # unsigned(0)
         64                             # text(4)
            43616E63                    # "Canc"
         F5                             # primitive(21)
         64                             # text(4)
            55706474                    # "Updt"
         F4                             # primitive(20)
         64                             # text(4)
            52737073                    # "Rsps"
         80                             # array(0)

Which is invalid, thanks to map(7) - i.e., here's a map with 7 fields, but I'm only going to give you 5 of them. Hence the empty Rsps array pulled up into the request map as if it's a field, but it's still missing one more field even then.

Basically, the dag-cbor codec relies on Length() when writing a map(x) token, and bidnode is just reporting how many fields in its associated struct, discounting any that may be optional and missing, and then the map iterator is only returning 5 of them.

ipld / go-ipld-prime

bindnode doesn't calculate map `Length()` properly when properties are absent #332