Fix yarn run error when script is empty string (#624)

* Fix yarn run error when script is empty string

lib/dependencies.sh

@@ -21,13 +21,18 @@ run_if_present() {
   local build_dir=${1:-}
   local script_name=${2:-}
   local has_script_name
+  local script
 
   has_script_name=$(has_script "$build_dir/package.json" "$script_name")
+  script=$(read_json "$build_dir/package.json" ".scripts[\"$script_name\"]")
 
   if [[ "$has_script_name" == "true" ]]; then
     if $YARN; then
       echo "Running $script_name (yarn)"
-      monitor "$script_name" yarn run "$script_name"
+      # yarn will throw an error if the script is an empty string, so check for this case
+      if [[ -n "$script" ]]; then
+        monitor "$script_name" yarn run "$script_name"
+      fi
     else
       echo "Running $script_name"
       monitor "$script_name" npm run "$script_name" --if-present

test/fixtures/empty-heroku-postbuild-yarn/README.md

+A fake README, to keep npm from polluting stderr.
\ No newline at end of file

test/fixtures/empty-heroku-postbuild-yarn/package.json

+{
+  "name": "node-buildpack-test-app",
+  "version": "0.0.1",
+  "description": "node buildpack integration test app",
+  "repository" : {
+    "type" : "git",
+    "url" : "http://github.com/example/example.git"
+  },
+  "scripts" : {
+    "build" : "echo build hook message",
+    "heroku-postbuild": ""
+  },
+  "heroku-run-build-script": true
+}

test/fixtures/empty-heroku-postbuild-yarn/yarn.lock

+# THIS IS AN AUTOGENERATED FILE. DO NOT EDIT THIS FILE DIRECTLY.
+# yarn lockfile v1
+
+

test/run

@@ -87,6 +87,14 @@ testPreferEmptyHerokuPostbuildOverBuild() {
   assertCapturedSuccess
 }
 
+testEmptyHerokuPostbuildWithYarn() {
+  compile "empty-heroku-postbuild-yarn"
+  assertCaptured "Running heroku-postbuild (yarn)"
+  assertNotCaptured "build hook message"
+  assertNotCaptured "Script must exist"
+  assertCapturedSuccess
+}
+
 testPrePostBuildScripts() {
   compile "pre-post-build-scripts"
   assertCaptured "Running heroku-prebuild"