overview: Hide the list of activities when there are none

This allows the app launcher to take all available space.
5 jobs for hide-activities in 6 minutes and 19 seconds (queued for 1 minute and 40 seconds)
latest
Status Job ID Name Coverage
  Build
passed #137643
librem5
build:native

00:01:49

 
  Test
passed #137645
librem5
test:smoke:one-output

00:02:19

passed #137646
librem5
test:smoke:two-outputs

00:02:16

passed #137644
librem5
unit-test:native

00:01:24

6.1%
 
  Package
passed #137647
librem5
package:deb

00:01:42