fix: ensure we can deploy code when there is no pre or post-deploy script defined

Closes #3385

tests/unit/40_app-json.bats

@@ -17,7 +17,7 @@ teardown() {
 }
 
 @test "(app-json) app.json scripts" {
-  deploy_app
+  deploy_app nodejs-express
 
   run /bin/bash -c "dokku run $TEST_APP ls /app/prebuild.test"
   echo "output: $output"
@@ -36,3 +36,11 @@ teardown() {
   echo "status: $status"
   assert_success
 }
+
+
+@test "(app-json) app.json scripts missing" {
+  deploy_app nodejs-express-noappjson
+  echo "output: $output"
+  echo "status: $status"
+  assert_success
+}