fix: bug when empty event headers (CodeGenieApp#462) (CodeGenieApp#463)

Due to in several express.js libraries treats req.headers as NonNullable Object,
when event headers is null causing bugs.

When incoming headers from API Gateway is null, it should transform to {}.

Fix CodeGenieApp#462

src/event-sources/utils.js

@@ -32,7 +32,7 @@ function getRequestValuesFromEvent ({
 
  if (event.multiValueHeaders) {
  headers = getCommaDelimitedHeaders({ headersMap: event.multiValueHeaders, lowerCaseKey: true })
- } else {
+ } else if (event.headers) {
  headers = event.headers
  }