No times for pre- and post- survey links

[carolbooth2]

Is your feature request related to a problem? Please describe.
It would be great to include links to pre- and post- surveys in the schedule, as in the original schedule provided by Carpentries. The schedule generator automatically adds times to all rows of the table but this isn't appropriate for 'before' and 'after' for surveys

Describe the solution you'd like
allow blank time at least in the lines at beginning and end of schedule

Describe alternatives you've considered
I attempted to remove timings during schedule editing but the application replaces the times on export.

Context

I have shown that this will automatically generate a link to the pre-survey:
<td><a href="{{ site.pre_survey }}{{ site.github.project_title }}" target="_blank" rel="noopener noreferrer">Pre-workshop survey</a> </td>
(and similar code for post-survey)
https://nclrse-training.github.io/2024-11-05-NCL1/
will become:
https://nclrse-training.github.io/2024-11-05-NCL