Fix: Don't overwrite the transaction on an event if it's already been set.

[ThatTobMate]

This PR prevents the transaction property being overwritten if it has already been set on the event.
v5.2.0 saw the introduction of the setTransaction function on the scopes . The value set using this function in @sentry/node isn't being respected currently.

Setting the transaction in @sentry/browser works fine.

Example issue

When I inspect the current scope after I've used the scope.setTransaction call I can see the transaction is set correctly, however if I check the event in the beforeSend callback it's been changed.

Transaction value from the current scope before submitting an error

Transaction value from beforeSend

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[andybriggs]

💥

[HazAT]

Thanks! 💯