APIs with null sharding tags shouldn't be deployed on Gateway with tags

[exalate-issue-sync]

Description

v4 APIs have highlighted an issue with their deployment on the gateways if they have sharding tags set to null.

To reproduce

Steps to reproduce the behaviour:

1. In your v4 environment, deploy a sharding tag to any gateways
2. Create a new v4 API with a Kafka backend. This can have any kind or number of plans at inception.
3. Call the newly created API, and it will return a 200.
4. Take an action that will cause the API to become out of sync and require redeployment (eg. create and publish a new plan); redeploy the API.
5. Call the API once more. This time, a {{404 No context-path matches the request URI }}will always be returned, and no actions taken will result in a 200 again, including restarting the gateway. Only adding a sharding tag to the API will resolve the 404.