test: assert empty raw vector-image and existent computed vector-image

After the report rename made --logs-global-vector-image return the raw
stored value (empty when unset) and split --logs-computed-vector-image
out as the default-resolved key, the (logs:report)
vector-global-image and vector-global-networks raw test still pinned
assert_output_exists on the raw key in the unset state. The raw key is
correctly empty after dokku logs:set --global vector-image runs, and
the bundled vector image default is exposed via the computed key now.

tests/unit/logs.bats

@@ -539,6 +539,10 @@ teardown() {
 
   run /bin/bash -c "dokku logs:report --global --format json | jq -r '.\"logs-global-vector-image\"'"
   assert_success
+  assert_output_not_exists
+
+  run /bin/bash -c "dokku logs:report --global --format json | jq -r '.\"logs-computed-vector-image\"'"
+  assert_success
   assert_output_exists
 
   run /bin/bash -c "dokku logs:set --global vector-image timberio/vector:custom"