Fix bug when deleting the last tag

When IncidenceEditor deletes the last tag of an incidence, it
calls `Item::setTags()` with an empty tag list, and eventually
`ProtocolHelper::entitySetToScope()` rejects the empty list and
returns an error.

BUG: 431297
FIXED-IN: 5.16.2
2 jobs for !46 with delete-last-tag in 20 minutes and 46 seconds (queued for 1 second)
detached
Status Job ID Name Coverage
  Build
failed #89044
build_clazy_clang_tidy

00:07:48

passed #89043
linux kf5-qt5 qt5.15

00:12:58

1.0%
 
Name Stage Failure
failed
build_clazy_clang_tidy Build
[ 31%] Linking CXX static library ../../lib/libakonadiserver.a
[ 31%] Built target libakonadiserver
make: *** [Makefile:160: all] Error 2
Uploading artifacts for failed job
Uploading artifacts...
WARNING: build/clang-tidy-report.xml: no matching files
ERROR: No files to upload
Cleaning up file based variables
ERROR: Job failed: exit code 1