[FIX] base: value_to_html doens't keep the minus sign for times between 0 and 1

- Install timesheets and studio.
- In timesheets add a time of -0.5 (minus half an hour).
- Enter studio
- Switch to the Reports tab, and click Timesheet Entries.

Before this commit:

The time is displayed as 00:30.

After this commit:

The time is displayed as -00:30.

closes odoo/odoo#38354

Opw: 2036188
Signed-off-by: Nicolas Martinelli (nim) <nim@odoo.com>