Fix nil subscription stats to return error

[tomhollingworth]

Fix nil subscription stats to return error

Bug Report

I'm connecting to KepwareEX V6 with v0.3.5 with a number of subscriptions and calling the experimental feature StatsWithContext(ctx). Calling StatsWithContext on a BadNodeId subscription is responding with a StatusOK and a nil *ua.Variant object which then results in a panic as it tries to cast and iterate a slice v.Value().([]*ua.ExtensionObject) see subscription.go line 311.

I expect if the value (v set on subscription.go line 306) is empty, it either returns an error stating such, or returns a nil *ua.SubscriptionDiagnosticsDataType but does not panic.

Fix

Add a nil pointer check before casting and iterating the array, returning an error message if the value is empty.

Review Notes

Is there anywhere to document error messages or a style guide for error messages?