GenericThreadStackManagerImpl_OpenThread has non-spec-compliant encoding of some fields

[tcarmelveilleux]

When doing a wildcard read of a Thread device implementing default ThreadDiagnostics cluster (GenericThreadStackManagerImpl_OpenThread), we get failures on reading/subscribing to several attributes in the cluster when not yet associated to a Thread mesh:

chip.clusters.Objects.ThreadNetworkDiagnostics.Attributes.ActiveTimestamp: ValueDecodeFailure(TLVValue=None, Reason=InteractionModelError(<Status.Failure: 1>)),
   chip.clusters.Objects.ThreadNetworkDiagnostics.Attributes.PendingTimestamp: ValueDecodeFailure(TLVValue=None, Reason=InteractionModelError(<Status.Failure: 1>)),
   chip.clusters.Objects.ThreadNetworkDiagnostics.Attributes.Delay: ValueDecodeFailure(TLVValue=None, Reason=InteractionModelError(<Status.Failure: 1>)),
   chip.clusters.Objects.ThreadNetworkDiagnostics.Attributes.SecurityPolicy: ValueDecodeFailure(TLVValue=None, Reason=InteractionModelError(<Status.Failure: 1>)),
   chip.clusters.Objects.ThreadNetworkDiagnostics.Attributes.ChannelPage0Mask: ValueDecodeFailure(TLVValue=None, Reason=InteractionModelError(<Status.Failure: 1>)),
   chip.clusters.Objects.ThreadNetworkDiagnostics.Attributes.OperationalDatasetComponents: ValueDecodeFailure(TLVValue=None, Reason=InteractionModelError(<Status.Failure: 1>)),

All the above are only valid when Thread is properly configured.
The code (simplified slightly for brevety) in src/platform/OpenThread/GenericThreadStackManagerImpl_OpenThread.hpp in GenericThreadStackManagerImpl_OpenThread<ImplClass>::_WriteThreadNetworkDiagnosticAttributeToTlv has, for example:

    case ThreadNetworkDiagnostics::Attributes::ActiveTimestamp::Id: {
        err = CHIP_ERROR_INCORRECT_STATE;
        if (otDatasetIsCommissioned(mOTInst))
        {
            otOperationalDataset activeDataset;
            otError otErr = otDatasetGetActive(mOTInst, &activeDataset);
            VerifyOrExit(otErr == OT_ERROR_NONE, err = MapOpenThreadError(otErr));
            uint64_t activeTimestamp  = activeDataset.mActiveTimestamp;
            err = encoder.Encode(activeTimestamp);
        }
    }
    break;

This will simply return a CHIP_ERROR (which will lead to generic IM error ("FAILURE") instead of writing a null like the spec requires:

11.13.6.57. ActiveTimestamp Attribute

This attribute SHALL be null when there is no dataset configured.

It appears all the problematic FAILURE are of that nature.

[bzbarsky-apple]

@jmartinez-silabs