Don't send signals when appID is empty

[winsmith]

Multiple apps are sending us events where appID = "" and TelemetryDeck.SDK.namespace = "". In that case, we should not send the event at all, since it can't be routed to a customer anyway.

[winsmith]

(This is not very urgent, but we could fold it into the v3 release, I think)

[kkostov]

@winsmith sounds good, in v3 we have the validation step available in the pipeline which will be perfect to cover this