fix: Ensure support for when no lambda function (serverless#842)

Author: junah201

lib/inject.js

@@ -99,6 +99,10 @@ function moveModuleUp(source, target, module) {
  * @return {Promise} the combined promise for requirements injection.
  */
 async function injectAllRequirements(funcArtifact) {
+  if (this.targetFuncs.length === 0) {
+    return BbPromise.resolve();
+  }
+
   if (this.options.layer) {
     // The requirements will be placed in a Layer, so just resolve
     return BbPromise.resolve();