fix: properly detect empty subcommands

Closes #2849

tests/unit/40_core_2.bats

@@ -50,10 +50,16 @@ build_nginx_config() {
   echo "output: "$output
   echo "status: "$status
   assert_failure
+
   run /bin/bash -c "dokku fakecommand 2>&1 | grep -q 'is not a dokku command'"
   echo "output: "$output
   echo "status: "$status
   assert_success
+
+  run /bin/bash -c "dokku apps: 2>&1 | grep -q 'is not a dokku command'"
+  echo "output: "$output
+  echo "status: "$status
+  assert_success
 }
 
 @test "(core) urls (non-ssl)" {