The data type of IdTime is 'Optional[float]' [1], [2]. Before this patch the
msgpack decoder could only decode 'IdTime' in case its data type was
'float'. Now it also supports the decoding of IdTime in case it is NIL.

Besides the changes in 'protogen.go', this patch also relaxes the
overflow check test in 'proto_test.go', which fails in its previous form
after adjusting the decoding. However I don't think the exact error
type should matter here: in real-usage cases we don't mind about the
particular error type [3].

[1] See here, the fifth argument is IdTime:

    https://lab.nexedi.com/nexedi/neoppod/-/blob/3ddb6663/neo/master/handlers/identification.py#L26-27

    This is found to be 'Optional[float]':

    https://lab.nexedi.com/nexedi/neoppod/-/blob/3ddb6663/neo/tests/protocol#L98

[2] This seems to be true for both, pre-msgpack and post-msgpack
    protocol, because in the pre-msgpack NEO/go there is already this
    note:

    https://lab.nexedi.com/kirr/neo/-/blob/1ad088c8/go/neo/proto/proto.go#L352-357

[3] See https://lab.nexedi.com/kirr/neo/-/blob/6fb93a60/go/neo/neonet/connection.go#L1531-1534
    and https://lab.nexedi.com/kirr/neo/-/blob/6fb93a60/go/neo/neonet/connection.go#L1588-1591