fix(central): Ensure only valid pending reason are taken into account to display or not the warning message

[ccailly]

Checklist before requesting a review

Please delete options that are not relevant.

• I have read the CONTRIBUTING document.
• I have performed a self-review of my code.
• I have added tests that prove my fix is effective or that my feature works.
• This change requires a documentation update.

Description

• It fixes Pending reason warning on homepage confusing users #21603

Currently, we display a warning message if pending reasons have been defined but notifications are not enabled.
However, the current system defines "empty" pending reasons when the user puts the ticket on pending using the toggle at the bottom of the followup or task form.
This fix strengthens the conditions for displaying the warning so that the reasons for waiting are considered "valid."

[cconard96]

If the pending reasons are invalid, is it safe to prevent them from being added to begin with?

IMO even with the invalid pending reasons being ignored, there are still some things that remain unaddressed for user experience:

• Is it important to have this kind of warning specifically for the auto_reminder notification when we don't do anything like this for other notifications?
• Should this notification be enabled by default?