[Bug]: Calendar event changed notification diff: a previously empty value is not visible

⚠️ This issue respects the following points: ⚠️

• This is a bug, not a question or a configuration/webserver/proxy issue.
• This issue is not already reported on Github OR Nextcloud Community Forum (I've searched it).
• Nextcloud Server is up to date. See Maintenance and Release Schedule for supported versions.
• I agree to follow Nextcloud's Code of Conduct.

Bug description

I a notification about a changed calendar event, overwritten values are supposed to be shown with a strike-through text-decoration. If the previous value is empty, e.g. because no location was set, then the previous value is empty, and thus not shown at all. Then there's no way to determine from the email was has actually changed.

The HTML part of the email contains <span style='text-decoration: line-through'></span>.

The text-part does not contain any hint on the previous value.

This is on cloud.nc.c from today (2025-02-07).

Steps to reproduce

1. Create an event with another participant, leave the location field empty.
2. Edit the event to contain some text in the location field.
3. Read the notification email of the additional partitipant.

Expected behavior

I would expect that the notification explains to me, what has changed. If a field was previously empty, it must somehow be highlighted as new (also in the text-part, please!).

I would also find it helpful if there was a summary, which fields did change, so I don't have to scan all fields for (maybe tiny) text changes.

Start from the calendar event change notification email generation and compare how the HTML and text parts represent an empty previous value. Reproduce the location change described in the issue; done means an empty-to-value change is clearly conveyed in both parts of the notification.