Fix (TicketTask) empty task duration dropdown no longer overrides the planned end date

[Herafia]

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 !45088
• Here is a brief description of what this PR does

The task form contains two duration fields that are submitted at the same time during saving:

The duration drop-down list: actiontime field
The schedule
Upon saving, handleTaskDuration determines which one takes precedence. Its rule was:

If the duration from the drop-down list differs from the one saved, then it takes precedence, and it recalculates the end date as start date + duration.

The duration drop-down list only offers fixed increments (1 to 9 minutes, then in 5-minute increments, then in hours, etc.). If the task has a duration that does not match any of these increments (for example, 12 minutes), the list cannot display it → it defaults to the empty option “-----,” which sends the value 0.

Upon saving:

list duration (0) ≠ saved duration (e.g., 720 s) → the list “wins”
therefore end = start + 0 = start
therefore start = end → validation fails → ERROR

Now: we only allow the “duration” list to override the end date if an actual duration (> 0) has been selected.

Screenshots (if appropriate):

[stonebuzz]

with requested change

[trasher]

Also, please rebase ti update CI conf