Fix bug where empty vpc connector setting was removed.

[taeold]

Today, empty vpc connector setting is removed in the function update request, e.g.:

exports.vpc = functions.runWith({ vpcConnector: '' }).https.onRequest((request, response) => {
  functions.logger.info("Hello logs!", {structuredData: true});
  response.send("Hello from Firebase!");
});

This is unfortunate since the existing behavior makes it impossible to clear vpc connector setting (see #3968 for discussion). The change here preserves the empty string as a vpc connector setting which in turn allows someone to clear the vpc connector setting on deploy.