optional fields

[pyramation]

match output from ts-proto as discussed in confio/cosmjs-types#40

[pyramation]

The proto of this is:

message ExistenceProof {
    bytes key = 1;
    bytes value = 2;
    LeafOp leaf = 3;
    repeated InnerOp path = 4;    
}

but nested messages should be nullable by default if I recall correctly. This is described in detail here: https://github.com/protocolbuffers/protobuf/blob/main/docs/field_presence.md.

The nullable = false option of gogoproto exists because nullable is the default for messages. But it also comes with a warning:

Warning about nullable: According to the Protocol Buffer specification, you should be able to tell whether a field is set or unset. With the option nullable=false this feature is lost, since your non-nullable fields will always be set. It can be seen as a layer on top of Protocol Buffers, where before and after marshalling all non-nullable fields are set and they cannot be unset.