gitlab-ci: Fix $TRIGGER_UPDATES check

Repeat with me: `test -e` stands for "file exists", *not* for "string is
empty", as that's `test -z`.

Update the test in `trigger-updates` accordingly to properly print the
help text when the variable is not set.

Signed-off-by: Emanuele Aina <emanuele.aina@collabora.com>

.gitlab-ci.yml

@@ -230,7 +230,7 @@ trigger-updates:
         wget --header "PRIVATE-TOKEN: $CI_JOB_TOKEN" "$CI_PROJECT_URL/-/jobs/$TRIGGER_FROM_JOB/artifacts/raw/packaging-updates.yaml"
       fi
     - |
-      if [ -e "$TRIGGER_UPDATES" ]
+      if [ -z "$TRIGGER_UPDATES" ]
       then
         echo 'Set TRIGGER_UPDATES by manually triggering the pipeline to actually initiate the updates'
         echo '* use "*" to match everything'