Metrics display `0.00 sec` time instead of `—`

Related to #71.

The blank state for the total time metric displays a zero value instead of a placeholder dash like the others. It should be consistent and display as "—" until a value is set.

<img width="631" alt="Screenshot 2023-12-06 at 15 26 11" src="https://github.com/replicate/llama-chat/assets/7659/939abc52-03e9-481f-a28f-dd28aea4b031">
