v1.3.3: allow undefined bodies with nice warning message

thanks to @danew for resolving this "good first issue": #91

If you don't return a body, we will warn you with this warning in non-production environments:

Your lambda function didn't return a body, which may be a mistake. Check our Usage docs for examples (https://github.com/netlify/netlify-lambda#usage). 
If this is intentional, you can silence this warning by setting process.ENV.SILENCE_EMPTY_LAMBDA_WARNING to a truthy value or process.env.CONTEXT to 'production'