Next.js: Fix "default" op for certain serverside transactions

[lforst]

Description

We haven't figured out why this happens yet but here are some examples of the transactions having a default op:

https://sentry.sentry.io/performance/trace/e7e80d610bf656ae8a0feaccd0385aa3/?dataset=transactions&eventId=7ed7154f69a64049bc4bd7df44e51445&field=title&field=project&field=user.display&field=timestamp&field=transaction.op&name=All%20Errors&project=4507657212592128&query=RSC&queryDataset=transaction-like&sort=-timestamp&source=discover&statsPeriod=1h&timestamp=1725537009&yAxis=count%28%29

It always seems to be for navigational transactions but also other cases:

https://sentry.sentry.io/performance/trace/938f2cafe045c5fb7ca8d61ea876ca69/?dataset=transactions&field=title&field=project&field=user.display&field=timestamp&field=transaction.op&name=All%20Errors&node=txn-f7765255066441cc8468b325ff76a8f8&project=4507657212592128&query=transaction.op%3Adefault&queryDataset=transaction-like&sort=-timestamp&source=discover&statsPeriod=1h&timestamp=1725536777&yAxis=count%28%29

https://sentry.sentry.io/performance/trace/4968396890d249e9f2e08a56bae52db1/?dataset=transactions&field=title&field=project&field=user.display&field=timestamp&field=transaction.op&name=All%20Errors&node=txn-887fe0cccfea4d2b8beb82c6803d3f60&project=4507657212592128&query=transaction.op%3Adefault&queryDataset=transaction-like&sort=-timestamp&source=discover&statsPeriod=1h&timestamp=1725536731&yAxis=count%28%29