🐛[RUMF-559] prevent event without sessionId

[bcaudan]

Motivation

Some events are sent to the intake without a session id.

Scenario to reproduce:

• sdk initialized with a session not tracked on two different tabs
• session expiration
• one tab renew the session in a tracked state
• generate some events on the second tab without interacting with it

then the second tab still considers to be on the last view of the first session, so viewContext.sessionId = undefined.
Events can be added to the batch because session.isTracked() = true due to the value of the cookie.

No matter what is the visibility state of the second tab, we don't want to create a new view and collect the associated events before any real user action on this tab.

Changes

Ensure that viewContext.sessionId is defined to allow events to be added to the batch.

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I have gone over the contributing documentation.

[mquentin]

Neat !