plausible-fix-top-pages-comparison

## Task

users have said they are seeing blank dates in row tooltip on the Top Pages report when using the comparison feature, looks like both date ranges are missing from the tooltip header. other reports (sources, channels, devices) display the dates fine in comparison mode. can you dig in and fix this?

## General instructions

- The code repo is at /repo/plausible.
- You are inside of a Docker container. You may not be able to perform all operations you would normally be able to do on a local machine. Dependencies have not been pre-installed, and you may need to install them yourself.
- You are expected to act autonomously as a software engineer to complete tasks you are given.
- Do not stop until you feel you have completed the task and your code changes can be merged.
- You may need to use software engineering skills like analyzing the codebase, researching technologies, running services, analyzing logs, etc. to complete the task. Not all tasks will be solvable by reading source code alone.