| !Time|<$view field="at">(no time specified)</$view>|
| !Participants|{{!!participants}}|

You might have also used a $transclude widget; in this case it's equivalent since there is no wikitext to wikify in the at field if we're using it normally. In Working with Dates, we'll learn that the $view widget has some special features for dates that usually make it more useful than the $transclude widget.

It's worth noting that TiddlyWiki here makes a distinction between an empty at field and an at field that doesn't exist at all: if the at field exists but is empty, it won't use the fallback. If you don't want there to be a difference between these states, use a filter with the has operator instead.