fix: property access on undefined exit span

[johann-crabnebula]

This basically fixes the error getting thrown on spans that don't have an exit yet.

Even though I have to say that the underlying code is really wonky in the first place (I added the question mark to "restore the original behavior":

The original line looked like this:

const width = scaleToMax([span.exits[i] - enter], allExits - allEnters)[0];

If we the span.exits array is empty it will pretty much resolve to something like:

undefined - number = NaN

And then it will

scaleToMax(NaN, number);
// -> 
(NaN / number) * 100 = NaN

Which looks hella ugly and should be revised in the future but does not throw an error.

The updated code:

const exited = span.exits[i]?.timestamp;

const width = scaleToMax(
    [exited - entered.timestamp],
    allExits - allEnters
)[0];

Knowing that exits can be empty it is easy to spot the previous error. When exits are empty we call the property .timestamp on undefined which is not allowed.

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