[FIX] web: color for calendar views without `color` attribute

The `color` attribute allows specifying a field by which the calendar
view will color the event entries in the view.

In views where this attribute is not defined, the event was instead
given the `text-white` class, which rendered the event invisible in the
calendar, save for a slight left-border.

This commit just use the 1st color class by default in such cases,
ensuring all events are visible.

opw-2145652

closes odoo/odoo#42185

Signed-off-by: Damien Bouvy (dbo) <dbo@odoo.com>