Currently, if the tip does not have any content set in step (happens a
lot in test tours), the tip is not displayed properly. This happens
because the height of the tip is set based on the tip content which is
not available in our case.

This commit fixes the behavior by providing a default content in such
cases saying "Click here to go to the next step.". It has two benefits
(in case someone is manually walking through a test tour):
- The tip is displayed properly.
- We can guide the devs with a friendly message.

task-2792218

closes odoo/odoo#87322

Signed-off-by: Quentin Smetz (qsm) <qsm@odoo.com>